Support custom header buttons layouts

[ilya-m32]

Closes #27

In this MR I've generalized the button structure and it is now possible to have custom layouts in window headers similar to what you can have in Gnome apps or classic Xorg WMs.

Obvious use case for this would be to have buttons moved to the left side (like Unity):

But other possible layout combinations should work, too:

or even something crazy like

Also should be extendable if we will (ever) have an app menu button.

I didn't have any experience in rust so far, thus feedback is more than welcomed.

[ilya-m32]

found few things to improve during self review, will move to draft for a short moment

[ilya-m32]

It's ready for checking now!

[PolyMeilex]

This looks awesome!
Just some minor comments about stylistic stuff.

[ilya-m32]

@PolyMeilex
Thanks for the feedback! I've made changes to the MR, should be good to go

[PolyMeilex]

Ok, looks great now, just one more comment that I noticed, and some more testing and this should be good to go!

[orowith2os]

Might I suggest utilizing Settings from the ashpd crate?

[PolyMeilex]

Sadly ashpd with zbus would probably pull in more dependencies then this whole crate. Which is a big blocker, for small decorations crate. It would probably be better fitted in winit where we could integrate it with event loop, and update settings on the fly rather than once on startup, but I'm not sure if heavy deps like zbus are welcome in winit either (I actually don't know, haven't asked around).

[PolyMeilex]

Thank you!