Allow showing non attached dialogs in the workspace thumbnails of the overview
Right now the workspace thumbnails displayed in the workspace switcher of the overview are not showing any skip-taskbar windows except modal dialogs. There's no good way to show non-attached dialogs in the window clones and to prevent issues with DND between the workspace switcher and the window clones (dragging a non-attached dialog from the workspace switcher and dropping it onto the window clones would make it vanish instantly) we just don't show them at all in the overview.
Since the purpose of the workspace thumbnails is to show a replica of the workspace, the thumbnails should include every window that's visible on the workspace when the overview is closed.