wayland: Rework Xcursor theme loading

Xcursor loading code has hardcoded search paths, in order to take into
account distros installing app data in a different location,
libwayland-cursor sets the ICONDIR to the icon directory computed based
on the install prefix.

However, that won't work with gitlab CI because it relocates binaries. A
more robust way to find cursors would be to use QStandardPaths to find
all the icon directories on the system.

Another advantage of using own cursor loading code is that it allows us
to reuse cursor images that are symlinks. For example, with
breeze_cursors, almost half of the files in the cursors directory are
symlinks.

The main disadvantage of this approach is that we would have to keep the
search paths up to date. However, on the hand, there are not that many
of them, e.g. ~/.icons, ~/.local/share/icons, /usr/share/icons,
/usr/local/share/icons. The last three are implicitly handled by the
QStandardPaths.