Show theme-default checkbox only when workbench theme actually changed

The restart dialog opened from the Appearance preferences page showed a
"Use as default theme" checkbox whenever theming was enabled, even when
the only change was an unrelated setting such as monitor-specific UI
scaling or the colors-and-fonts theme. Gate the checkbox on the workbench
theme having actually changed in this performOk() call.

Fixes #4003

Author: vogella

bundles/org.eclipse.ui.workbench/eclipseui/org/eclipse/ui/internal/dialogs/ViewsPreferencePage.java

@@ -508,10 +508,11 @@ public boolean performOk() {
 		boolean showRestartDialog = false;
 		String restartDialogTitle = null;
 		String restartDialogMessage = null;
+		boolean themeChanged = false;
 
 		if (isThemingPossible()) {
 			ITheme theme = getSelectedTheme();
-			boolean themeChanged = theme != null && !theme.equals(currentTheme);
+			themeChanged = theme != null && !theme.equals(currentTheme);
 			boolean colorsAndFontsThemeChanged = !PlatformUI.getWorkbench().getThemeManager().getCurrentTheme().getId()
 					.equals(currentColorsAndFontsTheme.getId());
 
@@ -546,7 +547,7 @@ public boolean performOk() {
 
 		if (showRestartDialog) {
 			String themeId = null;
-			if (isThemingPossible()) {
+			if (themeChanged) {
 				ITheme theme = getSelectedTheme();
 				if (theme != null) {
 					themeId = theme.getId();