Fix empty page tree in pre-filtered preference dialog

The native search field rework (PR #3925) overrode setInitialText to only
set the placeholder message and stopped maintaining the inherited
initialText sentinel. The pre-filter path addFilter() still wrote the hint
"type filter text" into the field, which was then applied as a real filter
pattern (since it no longer equalled initialText), matching no preference
pages and leaving the tree empty.

Keep the initialText sentinel in sync in the override and stop writing the
hint into the field in addFilter(). The search icon now signals the field's
purpose, so the "type filter text" placeholder is dropped; initialText is
kept only for the empty-filter check and the field's accessible name.

Fixes #4041

Author: vogella

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

@@ -126,9 +126,9 @@ protected Text doCreateFilterText(Composite parent) {
 
 		@Override
 		public void setInitialText(String text) {
-			if (filterText != null && !filterText.isDisposed()) {
-				filterText.setMessage(text != null ? text : ""); //$NON-NLS-1$
-			}
+			// No placeholder hint; the search icon already signals the field's purpose.
+			// initialText is still tracked for the empty-filter check and accessible name.
+			initialText = text != null ? text : ""; //$NON-NLS-1$
 		}
 
 		/**
@@ -140,7 +140,8 @@ protected void addFilter(ViewerFilter filter) {
 			getViewer().addFilter(filter);
 
 			if (filterText != null) {
-				setFilterText(WorkbenchMessages.FilteredTree_FilterMessage);
+				// Keep the field empty so the viewer filter narrows the pages; the
+				// hint text would otherwise be applied as a pattern and hide all pages.
 				textChanged();
 			}