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

## 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.