When loading the icon themes, we scan all XDG_DATA_DIRS directories. Up until now, we've stopped scanning the XDG data dirs as soon as we found the theme we're looking for.
However, the same theme may exist in multiple XDG data dirs, and they may complement each other. In this case, loading just the first instance is wrong, and results in icons not being found.
This patch fixes this, by always searching all XDG data directories. Each theme instance is treated as a separate theme, to ensure each instance's directories are all searched before another instance's directories, when loading icons.
Closes #17