Better support for primary colour change

[gileluard]

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:

[BlockyBlockling]

I would also love to have a dark icon or even multiple Icons available to select