PreferenceDialog: render filter as native search field

Give the preference dialog's filter field the full set of search-related
style bits (SWT.SEARCH | SWT.ICON_SEARCH | SWT.ICON_CANCEL) so it is
rendered as a native search field with a magnifier and cancel icon.

Also override setInitialText to expose the hint via Text#setMessage
only, instead of stuffing it into the field's content. The dialog
auto-focuses the filter field on open, so the old initial-text fallback
was never actually shown as a placeholder and forced users to delete
the hint before typing.

Author: vogella

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

@@ -119,6 +119,18 @@ protected class PreferenceFilteredTree extends FilteredTree {
 			super(parent, treeStyle, filter, true, true);
 		}
 
+		@Override
+		protected Text doCreateFilterText(Composite parent) {
+			return new Text(parent, SWT.SINGLE | SWT.BORDER | SWT.SEARCH | SWT.ICON_SEARCH | SWT.ICON_CANCEL);
+		}
+
+		@Override
+		public void setInitialText(String text) {
+			if (filterText != null && !filterText.isDisposed()) {
+				filterText.setMessage(text != null ? text : ""); //$NON-NLS-1$
+			}
+		}
+
 		/**
 		 * Add an additional, optional filter to the viewer. If the filter text is
 		 * cleared, this filter will be removed from the TreeViewer.