Show viewer percent and snap the viewer

There are two commits:

1. Harish's "Show resize status" is from !70 (closed), with the logic moved from ViewerContainer further down to OverlayStack. I removed a GLib.timeout_add(1000, self.__viewer_realization_done_cb, None) call from undock because it is calling self.__createNewViewer() which is doing that already.

2. A simplified version of Harish's "Snapping while resizing", which takes advantage of the fact that VideoWidget is a Gtk.Frame which allows controlling the size allocated to the child widget through do_compute_child_allocation. Thanks Harish for doing the initial research!

@harishfulara @thiblahute Could you please review and try it out.