layout: Show important actors above top_window_group

layout: Show important actors above top_window_group

The top_window_group is used for windows like popup menus, which should
appear above shell chrome like the panel.

Since we want important actors such as the screen keyboard or modal
dialogs to be shown above those windows, add their actors after adding
global.top_window_group to this.uiGroup and provide a new function
addTopChrome() to add important chrome above the top_window_group.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/917

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/358