gtk4-builder-tool validate: Allow parent types outside of gtk
Currently it isn't possible to use gtk4-builder-tool
to validate composite templates that derive a parent class that's not part of gtk.
Current output is of gtk4-builder-tool validate
is, if the parent class isn't part of gtk:
Failed to lookup template parent type {OBJECT_TYPE}
Would be nice allow gtk4-builder-tool validate
to validate also this type of composite templates.