Workspace: show attached modal dialog

Windows in the overview should be like they appear in the workspace,
including modal dialogs that are attached above them.
In addition, hiding the dialogs in the overview causes a flash as
dialog appears at the end of the transition.

Based on a patch by Maxim Ermilov <zaspire@rambler.ru>

https://bugzilla.gnome.org/show_bug.cgi?id=650843
WorkspaceThumbnails: show attached modal dialogs togheter with their parents

The window clones in the central part of the overview are showing modal
dialogs now, and this creates an inconsistency if the thumbnail doesn't too.
Code is intentionally similar in the two places.

https://bugzilla.gnome.org/show_bug.cgi?id=650843
WindowManager: WORKAROUND: disable dimming in the overview

Clones and effects don't mix, due to a bug in Clutter we don't
know how to fix, and sometimes the clone is clipped completely.
As a workaround, undim windows with dialogs in the overview.

Clutter bug: https://bugzilla.gnome.org/show_bug.cgi?id=659523

https://bugzilla.gnome.org/show_bug.cgi?id=650843