gui: unset GTK2_RC_FILES to prevent KDE from breaking our custom theming
(cherry picked from commit 67efdcc46c7f3673ef3d9bb4a51f9c9dd5579df2)
gui: remove redundant gtk_rc_parse
(cherry picked from commit fd4a15df50c317d4eeace11a42230c99abdbe5ab)
Next try to fix gtkrc parsing
(cherry picked from commit 99e38fd06983d9ba8342c40daf5f772059c31e30)