Don't crash on monitor remove

[scorpion-26]

gBar no longer crashes when it's monitor is removed and is instead evacuated onto another monitor or the next monitor added. We don't change any monitor indices, etc. yet, which results in some buggy results.

TODO:

• [x] Update layout
• [x] Update monitor indices for ws
• [x] ~Only re-init on the same monitor~ Snap back to the target monitor if available

Full implementation fixes: https://github.com/scorpion-26/gBar/issues/30 https://github.com/scorpion-26/gBar/issues/19

[scorpion-26]

This is now finally done. The way it works now, is that the Window has a "target monitor" (The one specified at the start), where the Window will stay or snap back if available. If the target monitor is not available, the Window will open on any other available monitor (except a headless monitor). There are probably still many bugs with the way we translate from the old monitor id system (with index; the only way to get a GdkMonitor) to the new way (with connector name; used internally to track the current and target monitor), though I couldn't find any bugs left myself.