gtk backend should switch to gtk.Builder

[galtgendo]

This is both simple and complicated.
After using the automatic conversion from libglade to gtkbuilder format on lineprops.glade, the initial changes to the code are just about trivial.

The catch lies in the details:

• first thing of note: unless I'm missing something, there's no libglade introspection, therefore gtk3 backend needs this change;
• GtkBuilder is gtk 2.12 minimum (same for pygtk) - I don't see this as a problem (pygtk 2.14 is only slightly older than python 2.6), YMMV
• gtk3 has no text-only API on GtkComboBox, it has GtkComboBoxtText instead; it was added in gtk+ 2.24, however even though pygtk docs claim it's available in that version, it seems that somebody didn't test the changes, cause it doesn't work there - in gtk3 it sort of works, but will need a little tweak - with pygtk being effectively abandoned that's a big hurdle

[tacaswell]

@fariza Do you have a comment on this (as you are hacking at the gtk stuff right now)?

[fariza]

Personally I dont see the need, the GUI is too simple.

The flexibility is not really needed here and as @galtgendo pointed out, the Gtk2 vs Gtk3 problems makes it more complicated.

Maybe I'm missing the point....