xdg-shell: Rewrite wrappers for the xdg-shell protocol

The current xdg-shell wrappers don't match existing abstractions in the
xdg-shell protocol well, which makes it more difficult to refactor code
that is responsible for managing configure events and geometry in kwin.

Given that the xdg_decoration and the xdg_shell protocols are tightly
coupled together, I had to rewrite our wrappers for the xdg_decoration
protocol as well.