kconfig: gtk interface: fix splitview, split code into two files

Split code into gconf.c and gconf_treeview_model. gconf.c now
only contains ui init and ui signal callback code, all tree/model
code is in the second file.

Issues with split view are fixed.
kconfig: images.c for gtk/qt gui flavour reoganized

Change order of xpm image constants to build up groups.
Make those groups optional via ifdefs.