Rework Xcursor loading code

Currently, Xcursor themes are loaded via libwayland API, which looks awkward because of the way how kwin communicates with itself via the internal connection. Most compositors nowadays either use libXcursor directly or maintain a copy of libXcursor's code that is responsible for loading Xcursors. Maintaining a fork of libXcursor's code isn't ideal, but I think it's not that terrible.

The main motivation behind this change is to limit the number of usages of kwayland client api.