Breakage occurred because tabItem reparented an item that had a
non-standard gDk parent window.

Solution: fix gdk window parent for such special controls.
(I.e ControlEditors in tables).

Verification:
- Open child eclipse, change method signature.
  Now table cells can be edited via click.
- Open attached snippet, with patch cells can be edited properly.
- AllTests Gtk3.22
- X11/Wayland.

Patchset 3:
- Minor rename of methods to clarify that it's a *gdk* window,

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=5108033
Change-Id: I8dcaac950eb0847dd97016b2140c607012550d2f
Signed-off-by: Leo Ufimtsev <lufimtse@redhat.com>