Add pointer grab for Wayland

[Cherser-s]

pointer_constraints_v1 extension has been around for almost a year. It is already supported in wlroots.

[elinorbgr]

This will need to go through Smithay/client-toolkit#72

[Cherser-s]

Does this functionality need to belong there? Seat manager isn't implemented in client-toolkit for example.

[elinorbgr]

SCTK goal is to provide general abstractions facilitating the use of the various wayland protocol extensions. The seat manage is not provided by it because it contains mostly winit-specific code, but it uses SCTK for handling the translation of keyboard events for example.

In the same approach, I expect the support of pointer-constraints and relative-pointer will caontain both winit-specific code and code that may be re-used in other apps. The latter part could live in SCTK to improve re-usability and simplify maintenance.

[Cherser-s]

Ok, but there is probably no need to implement relative pointer extension in SCTK, except for modifying Environment struct.

Also, adding relative pointer will require DeviceEvent queue. Now only WindowEvent queue is implemented. I've simply added DeviceEventsSink Cherser-s@eabe813.

[elinorbgr]

Well, I hadn't really started to look into these protocols so this was kind of a generic response. If you already have a working implementation, this will be easier to discuss with a PR though.

[Cherser-s]

Ok, I wrote implementation yesterday evening though.