Draft: Allow themes to define external margin for dialogs

Like I was considering for the panel, this allows themes to define an external margin (I think I called "floating" here, it's just a draft, I'd prefer a better name) to make them floating.

Differently from the panel testing I was had done, here I choose not to preserve fitts law. I thought about it and I felt like it could bring up issues (I can't assume where dialogs are used, so I don't want them extending over other elements) plus the code complexity was going to be very high and it was not worth it.

This patch aims to be super simple yet work.

For some reason this breaks sidebars, I need to investigate. It also picks up notifications, ops. I need to come up with a more specific way of restraining the dialog position.