Introduce a BrightnessManager

[leolost2605]

Make it build by making sure vala first includes meta/display which contains the necessary macros for the others. I will properly fix this by upstreaming the missing header includes.

[leolost2605]

@garaevdi if you want to take a look :) I fixed the build issues for now by forcing vala to use a certain include order

This follows the gnome shell implementation a bit (mostly regarding to using logical monitors and the global scale calculation which will be used by keybinds to change all monitor brightnesses at the same time while keeping the ratio between the monitors)
Needs still some work and the keybinds are missing but everything else is working already

[garaevdi]

@garaevdi if you want to take a look :) I fixed the build issues for now by forcing vala to use a certain include order

Oh, that's smart, didn't know about this trick 😅️

At a glance your implementation looks much cleaner, I'll close my PR. Just a suggestion: how about implicitly placing primary monitor at array's start, so interface consumers could highlight it somehow?