Having a comprehensive glade file check probably isn't worth our time, since
it'd be a moving target and the XML schema is mostly undocumented and glade is
supposed to check for most problems anyway, but I figured we could have a check
for the things we actually mess up.
I remember an issue a while ago where a file had duplicate IDs, and we have an
issue where mnemonic_widget properties reference objects that don't exist, so
check for those things. Feel free to add your own!