Switch to GtkBuilder

For the record, xlgui/prefs/widgets already supports plugin prefs in both Libglade and GtkBuilder formats. If you do a check on the file extension (.glade vs .ui) it should be easy enough to support both, if only to smooth the transition temporarily, given that they follow very similar API.

I meant xlgui/prefs/__init__, sorry. The relevant code is inside the switch_pane function. It checks whether the page has a 'ui' or 'glade' attribute, but checking the file extension would be just as easy.

> I meant xlgui/prefs/__init__, sorry. The relevant code is inside the switch_pane
> function. It checks whether the page has a 'ui' or 'glade' attribute, but checking
> the file extension would be just as easy.

It's probably better to leave it as-is, file extension based detection is much more fragile than simply allowing the developer to specify which format to use.