#882 Make choice of gtkrc-2.0.mine also dependent on $GTK2_RC_FILES

[Philipp Joram]

lxappearance locates gtkrc using $GTK2_RC_FILES. It would be useful to have additional user config that is included before any settings to be located in the same directory as gtkrc.

My reason for this suggestion is that if the user has set $GTK2_RC_FILES, they want to control where their config files go. I wouldn't make sense to hard code the path to ~/.gtkrc-2.0.mine, imo.

This is related to bug #822.