Add basic window control

[melix99]

This is the first merge-able part of #606.

It allows to implement custom CSDs via window management signals and a new WindowHandle widget which is useful to implement a custom titlebar.

[DJMcNab]

I'm happy with the additional signals; less sure about the new widget

[melix99]

Well, I guess I should've included more information on how the WindowHandle is supposed to work, so that it's clearer what my intentions are with this PR.

The WindowHandle is a widget area that allows you to move the window by dragging with your mouse. It's also a single-child container, so you are expected to put the content of the titlebar in there (title label, buttons, menus, etc.) . It's important to know that it's not supposed to be a titlebar by itself, you are still supposed to implement your own titlebar widget that wraps the WindowHandle. This way this widget is completely flexible on the titlebar design you want to implement.

[DJMcNab]

I still feel that WindowHandle as in this PR is poorly motivated; it seems hard to use correctly (given how anemic our other layout control is, and how little thought has been put into event bubbling; see #masonry > Missing event set_handled calls). Additionally, it's not used at all in this repository; I'd want to at least see an example.

My suggested next step here is to make this PR only the changes required to allow creating this WindowHandle externally (i.e. the changes except for the new widget), which we can easily land. I'd then like to see WindowHandle added in a more complete form.

[DJMcNab]

Apologies, this slipped out of my queue. This looks great, thanks.

Luckily, no merge conflicts seem to have arisen in the intervening period. Let's hope that's also true for CI :)