Align dark theme form chrome with regular view styling

The dark theme rendered the form heading and section title bars
indistinguishably from the form body, and editor-form text contrast
was noticeably worse than the dialog labels (#3945).

Bring the dark theme form chrome in line with the existing
".MPart Composite" / ".MPart Section" view styling and with dialog
label contrast:

- e4-dark_globalstyle.css: split the joint "Form, FormHeading" rule.
  Use #2F2F2F for both Form and FormHeading background-color (and the
  Form's text-background-color), matching the ".MPart Composite" body
  color used in views, and use #f4f7f7 for the FormHeading foreground
  for near-white dialog-level contrast. Drop the "swt-background-mode:
  'force'" hack on Form: it forced FormHeading children to inherit
  Form's body color, which is what required a counter-override in the
  partstyle. The light theme has never set this and works fine with
  INHERIT_DEFAULT.
- e4-dark_partstyle.css: remove the ".MPart FormHeading > ..." block.
  It was only needed to undo the FORCE inheritance and to hard-code
  #505f70 on the heading; with FORCE gone and the heading colors set
  in globalstyle, the block is redundant.
- e4-dark_win.css: change the Windows-specific ".MPart Form" block
  from #4f5355 to #2F2F2F so Form widgets on Windows align with the
  rest of the dark theme body color.

With these changes the previous editor-only override block
(".Editor Form Composite, .Editor Form Composite Tree,
.MPartStack.active .Editor Form Composite Tree { #1E1F22 }") in
e4-dark_{linux,mac,win}.css is no longer needed: ".MPart Composite"
already sets the form body to #2F2F2F, ".MPart Form Section Tree" sets
the section trees to #313538, and FormHeading is matched directly by
the FormHeading selector in globalstyle. Removing the override also
makes editor section trees match view section trees, which the broad
rule was inadvertently flattening.

Author: vogella

bundles/org.eclipse.ui.themes/css/dark/e4-dark_globalstyle.css

@@ -194,27 +194,24 @@ WebSite {
     color: #dddddd;
 }
 
-Form,
-FormHeading {
-    background-color: #505F70;
+Form {
+    background-color: #2F2F2F;
     color: #9AC9D8;
-}
 
-Form {
 	/* Bug 465148: Additional styling for the Form */
-    text-background-color: #505F70;
+    text-background-color: #2F2F2F;
 
 	tb-toggle-hover-color: #313538;
 	tb-toggle-color: #313538;
 	h-hover-full-color: #313538;
 	h-hover-light-color: #313538;
 	h-bottom-keyline-2-color: #313538;
 	h-bottom-keyline-1-color: #313538;
+}
 
-	/* We also have to force the background mode (the
-	 * Label/ToolBar in the heading should inherit it).
-	 */
-    swt-background-mode: 'force';
+FormHeading {
+    background-color: #2F2F2F;
+    color: #f4f7f7;
 }
 
 Section {

bundles/org.eclipse.ui.themes/css/dark/e4-dark_partstyle.css

@@ -94,22 +94,6 @@
     color: #BBBBBB;
 }
 
-.MPart FormHeading,
-.MPart FormHeading > TitleRegion,
-.MPart FormHeading > TitleRegion > Label,
-.MPart FormHeading > TitleRegion > StyledText {
-    background-color: #505f70;
-    color: #9ac9d8;
-}
-
-.MPart FormHeading,
-.MPart FormHeading > TitleRegion {
-    swt-background-mode: none;
-}
-.MPart FormHeading > CLabel {
-    background-color: #505f70;
-}
-
 .MPartFormHeaderCLabelError {
 	color:'#org-eclipse-ui-workbench-FORM_HEADING_ERROR_COLOR';
 }

bundles/org.eclipse.ui.themes/css/e4-dark_linux.css

@@ -112,13 +112,6 @@ ImageBasedFrame,
     swt-selected-highlight-top: true;
 }
 
-.Editor Form Composite,
-.Editor Form Composite Tree,
-.MPartStack.active .Editor Form Composite Tree
-{
-	background-color: #1E1F22;
-}
-
 #org-eclipse-e4-ui-compatibility-editor Canvas,
 #org-eclipse-e4-ui-compatibility-editor Canvas > *
 {

bundles/org.eclipse.ui.themes/css/e4-dark_mac.css

@@ -98,13 +98,6 @@ Button  {
     swt-selected-highlight-top: true;
 }
 
-.Editor Form Composite,
-.Editor Form Composite Tree,
-.MPartStack.active .Editor Form Composite Tree
-{
-	background-color: #1E1F22;
-}
-
 #org-eclipse-e4-ui-compatibility-editor Canvas,
 #org-eclipse-e4-ui-compatibility-editor Canvas > *
 {

bundles/org.eclipse.ui.themes/css/e4-dark_win.css

@@ -101,8 +101,8 @@ ImageBasedFrame,
 .MPartStack.active .MPart Form DependenciesComposite,
 .MPartStack.active .MPart Form ListEditorComposite,
 .MPartStack.active .MPart Form Text[style~='SWT.READ_ONLY'],
-.MPartStack.active .MPart Form DependenciesComposite > SashForm > Section > * { /* Section > DependenciesComposite$... */ 
-	background-color: #4f5355;
+.MPartStack.active .MPart Form DependenciesComposite > SashForm > Section > * { /* Section > DependenciesComposite$... */
+	background-color: #2F2F2F;
 	color: #f4f7f7;
 }
 #org-eclipse-help-ui-HelpView Form,
@@ -180,13 +180,6 @@ ImageBasedFrame,
     swt-selected-highlight-top: true;
 } 
 
-.Editor Form Composite,
-.Editor Form Composite Tree,
-.MPartStack.active .Editor Form Composite Tree
-{
-	background-color: #1E1F22;
-}
-
 #org-eclipse-e4-ui-compatibility-editor Canvas,
 #org-eclipse-e4-ui-compatibility-editor Canvas > *
 {