fix GObject.registerClass not allowing manually loaded templates
as per https://gitlab.gnome.org/GNOME/gjs/-/blob/master/examples/gtk4-template.js, the Template
can be a:
- a
Uint8Array
orGLib.Bytes
of XML - an absolute file URI, such as
file:///home/user/window.ui
- a GResource URI, such as
resource:///org/gnome/AppName/window.ui