Cursor theme should search in dirs specified by $XDG_DATA_DIRS if present

[jinliu]

Currently only $HOME/.local/share and /usr/share are searched. This wouldn't work for KDE devs, as the KDE dev build installs to $HOME/kde/usr by default, and adds $HOME/kde/usr/share to the front of $XDG_DATA_DIRS.

[vaxerski]

$XDG_DATA_DIRS is user-specific if I understand correctly... so we'd search themes in $XDG_DATA_DIRS/icons?

[Loara]

$XDG_DATA_DIRS is user-specific if I understand correctly... so we'd search themes in $XDG_DATA_DIRS/icons?

$XDG_DATA_DIRS should be system-specific, $XDG_DATA_HOME is instead user-specific.