WIP: Output management take 2

[flaviozavan]

Getting this out early, as it might benefit others. Builds on top of #79 . It's not perfect yet, but is currently stable enough for someone with two displays to alternate between them with confidence and hopefully no crashes. Trying to use more than one at once is very buggy still, but is functional enough to run wlr-randr or wdisplay to switch to only one..