Improve keyboard navigation in overview

This allows to switch application by keyboard only

• fixes focus in the carousel overview
• Converts the global keybindings to use GAction so we can register from different parts of the shell
• binds showing the overview

TODO to unwip: read 'show overview' keybinding from GSettings and after allowing more keys in phoc

/cc @devrtz since it touches the keybindings code

Closes: #175 (closed), #398 (closed)