Render shadows for half-tiled windows

Without this, side-by-side half-tiled windows are hard to discern.

https://bugzilla.gnome.org/show_bug.cgi?id=708857
Avoid a critical

gtk_window_get_focus can return NULL, deal with it.

https://bugzilla.gnome.org/show_bug.cgi?id=740812
GtkStackSwitcher: Make the buttons fill vertically

It fix some cases with different height for the buttons,
like with Hebrew character.

https://bugzilla.gnome.org/show_bug.cgi?id=740642
inspector: Simplify some code