gnome: (config) use text combo boxes

This switches the combo boxes type in videoconf.c from
GtkComboBox to GtkComboBoxText.

This allows us to get rid of the store, tree, iterator and text renderers
for each combo box, as well as the get_active_text() and
set_combo_index_from_str() helper functions.

Change-Id: I0886cd6df41da2e245d8c059efd96f6e452890db