Allow user to set what monitor a window is on through the `OnMonitor` Relationship

[Sapein]

What problem does this solve or what need does it fill?

This provides a nice little API that makes it easy to just move the Window to a Monitor through the OnMonitor Relationship, which might be expected.

What solution would you like?

Ideally inserting or otherwise changing the OnMonitor Relationship to point to a different Monitor then the Window should be moved there.

What alternative(s) have you considered?

We could just not do this.

Additional context

I was asked if my crate (that this functionality is based on) is Read Only or if updating the relationship would update the Monitor the Window is on, and while there is an existing API doing this could allow for some simpler code when doing something like a Settings Menu, as you're already needing to use the Relationship there anyways.

[alice-i-cecile]

I quite like this idea, but expect that this implementation will be harder than expected. Feel free to prove me wrong though!

[Sapein]

Yeah, that's how I felt as well.

One thing is that there is currently, Window.postition, which can also be used to set/change the Window's Position which is also how you change the Monitor a window is currently on. Which leads to the question of: "If a Window has its Monitor changed through this, and also through position which takes effect? Or would being able to set the Monitor a Window is on through position be removed? I'm not 100% certain what the best move here would be, but it's something I'll be thinking about at least.