glade files are converted mechanically with gtk-builder-convert with
no further changes to them in this commit on purpose, except for splitting
those with multiple dialogs described in them into separate files as needed
by common GtkBuilder usage. That splitting naturally solves all the
'WARNING: duplicate id "..."' messages from the automatic conversion as well.

mpp-bugzilla.glade is deleted as unused since first visible commit.

Resources are loaded with purely glib API - we'll be able to do this more
straightforward with gtk3 later with gtk_builder_new_from_resource.