Improve keyboard navigation for popup menus
A bunch of improvements to popup menus and panel menus:
- Make every popupMenu keyboard navigable and stop adding events to the source actor to allow opening it.
- Allow keyboard navigation of popup menus also if the popup was opened using a mouse
- Don't focus the first menu entry of the window menu by default, allows removing a workaround
- Reset focus after the pointer left the menu
- Allow closing panel menus by pressing the up arrow key
When this is merged we don't need !229 anymore.