Progress towards clean clang-tidy

This is some early work for making the clang-tidy linter pass in the future as stated in #17 (closed).

The largest change is a refactor of our wl_resource to internal type static conversion function. The function fromResource(wl_resource*) is replaced with handle(wl_resource*) which always returns the handle variable.

So we can use the same function for global binds as for other resources and we don't need any static casts (what fixes some clang-tidy checks).