SCTK API design reflexion

[elinorbgr]

Had been thinking about it for some time, so writing all this here for potential feedback.

Up to now, my main drive for SCTK was using it to implement winit's Wayland support. This has affected SCTK's development, in that I never really took time to do serious API design.

Currently, SCTK is just a pile of loosely coupled helpers to simplify writing Wayland apps, rather than an integrated toolkit. I've recently decided to reduce my involvement in winit and focus back on Smithay and SCTK. Following this, I'd like to work into making SCTK into a proper toolkit for writing Wayland apps.

This issues will serve to collect design ideas in general, I don't have a lot to put in yet and any input of people using or willing to use SCTK would be welcome.

[elinorbgr]

With the release of wayland-rs 0.24, wayland-client's API has significantly changed and SCTK will need profound changes to be ported. This is the ideal time to rethink & adapt the API as well as possible.

[elinorbgr]

This culminated as #85 .