Better support for primary colour change

Changing primary colour in Element still includes to modify some assets.

The following assets should be templated as much as we can:
camera_capture
camera_video_capture
placeholder
remove_icon

All FAB, actions button should have the same templated background (`placeholder.png` or programmatically drawn) and the icon (templated as well) as image. Ideally, we could create a specific class for such kind of button.
This corresponds to the following assets:

Also, files icon could be handled by a specific View class with 3 layers:
- the background (templated?)
- the frame templated
- the icon templated as well

This corresponds to the following assets:

Finally the favourites empty screen image may be split so we can template at least the star: