Remove input structs

Looking at the functionality in the server library, it's clear (in
hindsight) that there are two different "things" in there: 1) The IPC
API, that is, everything that concerns wl_display, wl_client,
wl_resource and 2) and half-hearted attempt at sharing input code and
focus logic that leaves a lot of problematic structs in the API
surface, only to share less than 1000 lines of code.

We can just move those input structs and helper functions into weston
and cut libwayland-server down to just the core server side IPC API.
In the short term, compositors can copy those structs and functions
into their source, but longer term, they're probably better off
reimplementing those objects and logic their native framework
(QObject, GObject etc).