Add layer system

[MariusDoe]

As a user, I want to create multiple composable, stacked layers in order to work on different parts of my painting independently.

[RogerAK]

• [x] Click the eye to show and Hide (@DieKautz @RogerAK) (4)
• [x] Rename as an extra button (@DieKautz @RogerAK) (1)
• [x] Fix eraser size when switching layers (@DieKautz @Encotric) (3)
• [x] Clean ViewPort (@kbroschke @MariusDoe) (5)
• [x] Write layer tests?