automatically toggle between docked and undocked mode

This is mostly to

• help testing convergence without having to fiddle with gsettings
• exploring how ui should look like
• give as enough leverage for Apps_Issues#159

See https://social.librem.one/@agx/105000536376844564

TODO

• better docked detection (currently we just count screens)
• figure out if this should stay in the shell or in a g-s-d plugin (lets keep it in the shell atm)
• a nice icon to indicate 'docked' mode (maybe @tobias.bernard has an idea)

This is what gets changed in docked mode:

• windows become floating
• gtk is toggled out of phone mode
• osk gets hidden
• headerbar switches to default layout
• icon is put into the status bar to indicate docked mode

Closes: #366 (closed)