cursor model: look for cursors in correct place

plasma-desktop's cursor theme kcm does the right thing, by consulting
the libXcursor library for the right search paths. Unfortunately, the
kcm here does a pretty butchered job of it. So, we copy, more or less,
the same algorithm used by plasma-desktop. Now there's parity in cursor
selection.

For reference, please see line 165 of:
https://quickgit.kde.org/?p=plasma-desktop.git&a=blob&f=kcms%2Fcursortheme%2Fxcursor%2Fthememodel.cpp