Mobile Shell feature

[TristanCacqueray]

The current default shell is Desktop: an app in charge of running other apps. It uses WinBox.js and a static tray bar which is not a good user experience for mobile users. Instead, butler should provide a new shell using this design:

┌───────────┐
│           │
│           │
│           │
│    App    │
│           │
│           │
│           │
│           │
├─────┬─────┤
│ NEW │ SEL │
└─────┴─────┘

That way, only a single app is visible by default. Here are some further improvements:

• Handle swipe motion to switch between apps.
• Allow spiting the screen to render two apps side by side.