Allow out-of-tree "page" objects to be accessible
Currently we only have in-tree "page" objects, but external libraries (like libadwaita) may want to make their own out of tree objects accessible just as well.
The main blockers are:
GtkATContextimplementation handles page objects specially
This means we're going to need to make the accessibility backend code slightly less "special", to allow out of tree objects to be handled as in tree ones.