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.

Fixes #98

Depends on !650 (closed), mutter!706 (closed)