Finish layer name editing

Keyboard hook solves the "disappearing Enter key" problem, so this
should be pretty solid now!
Optimize image compositor

The central image compositor (the thing that generates flat versions of
layered images) now leans on mergeTwoLayers to do all its heavy lifting.
This means that I can add new features to mergeTwoLayers (e.g. blend
modes), and have the benefits automatically extended to all compositing
operations throughout the program.