Move UI code to shell helper

[plfiorini]

Move user interface to shell helper

Benefits:

• More stability: restart on crash
• The compositor does what it's better at
• Use hardware layers
• Render only what needs to be rendered

While QtWayland Compositor has the appropriate infrastructure for hardware layers, there is currently no implementation for DRM.

In the future we will also be able to be clever and paint scene graph nodes differently, depending on the corresponding surface and try to squeeze the maximum efficiency out of the graphics chip.