Find/replace overlay: replace shell with integrated composite eclipse…

…-platform#2099

The FindReplaceOverlay is currently realized as a separate shell (more
precisely, a JFace Dialog), which is placed at a proper position on top
of the workbench shell. This has some drawback:
- It has to manually adapt to movements of the parent shell or the
target part/widget
- It has to manually hide and show depending on visibility changes of
the target part/widget
- It does not follow events of the target immediately, i.e., movements
are always some milliseconds behind, minimize/maximize
operations/animations are not synchronous etc.
- It does not locate properly when the platform uses Wayland, as manual
shell positioning is not possible there

This change replaces the dialog-based implementation of the
FindReplaceOverlay with an in-place composite-based implementation. A
composite is created in the target widget and placed relative to this
composite. In consequence, the overlay automatically follows all move,
resize, hide/show operations of the target widget.

Fixes eclipse-platform/eclipse.platform.swt#1447

Fixes eclipse-platform#2099

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/findandreplace/overlay/FindReplaceOverlay.java

@@ -43,7 +43,6 @@
 import org.eclipse.swt.widgets.ToolItem;
 import org.eclipse.swt.widgets.Widget;
 
-import org.eclipse.core.runtime.Platform.OS;
 import org.eclipse.core.runtime.Status;
 
 import org.eclipse.jface.bindings.keys.KeyStroke;
@@ -341,9 +340,6 @@ private void retrieveBackgroundColor() {
  }
 
  private Color retrieveDefaultCompositeBackground() {
- if (OS.isLinux()) {
- return widgetBackgroundColor;
- }
  AtomicReference<Color> colorReference = new AtomicReference<>();
  Dialog dummyDialogForColorRetrieval = new Dialog(targetControl.getShell()) {
  @Override