Jonas Dreßler authored May 03, 2019 and Florian Müllner committed May 15, 2019

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 #917

!358