Add confine pointer

[Cherser-s]

Resolves #72. It doesn't look good right now, and I didn't add the ability to grab pointer for new seats that can be added after pointer grab yet, since it requires providing new_seat method with surface reference.

[elinorbgr]

Hi, thanks for working on this!

Before all, could you explain me why you merged this logic with the Frame implementations, rather than creating a new module with this specific logic? Is there some coupling between them?

[Cherser-s]

Well because frames (and not window module) are holding references to WlPointer right now, so I thought that would be more convenient.
So I think to add some kind of get_pointers method to Frame module instead. (still requires updating pointer grab on new seat in case if new seat is created)
Also, I don't know whether it's better to constrain all available seats to surface at once, or constrain only specified one.

[elinorbgr]

A SCTK user would normally also have a WlPointer at hand without using the one internal to the Window/Frames, and separating the two would make for a more flexible API overall I think: users may wish to confine the pointer to some other surface than the one backing a Window.

In this approach, I guess we could have some API allowing the app to constrain a given pointer to a given surface, something like that.

[Cherser-s]

Ok, so Environment should contain an module for pointer grab (which contains a pointer_constraints object from the start and methods for both pointer confinement and lock).

[elinorbgr]

I think ideally this code should be separated into its own module (like the data_device, keyboard or pointer modules). Though this does raise some questions of API design for SCTK I had in mind for some time. Will make an issue about that.

[Cherser-s]

In this case there is no need to even implement API for that purpose since all you need to do is call one method (whether it is lock_pointer or confine_pointer). Client does only need to hold reference to created object to destroy it if needed. Or maybe just add wrapper for it to support multiple versions. (just in case API changes later since it is unstable by now)

[elinorbgr]

I'm going to close this, given this appear to need more design thought. Please head to #77 if you want to discuss this further. :)