Panel rework

This is my project to gradually make Cinnamon's panels more flexible. Instead of limiting the set of possible panel layouts to a few confusingly-named options, it opens up for user-defined layouts, with panels on different monitors.

The implementation is mainly backward-compatible with the current settings keys, except for the key controlling the panel layout, which had to be changed to enable switching back and forth between old and new versions.


This is a new version of pull #1504, the discussion of which got side-lined by people picking on petty details.

Current status: Ready to be merged.