Refactor layer panel

[barklund]

Feature Description

The layer panel should be refactored and moved to a more appropriate place in the folder structure.

Goals of this issue include:

• Make sure the layer/layer.js file is much smaller. Currently it sits at 500+ lines and is quite impossible to work with. There are also several bits of duplicated logic in there.
• Move the whole layer folder to a more appropriate location. The layer panel is no longer associated with the design panel nor the sidebar, so it should be decoupled from these in the folder structure.