view: Cast exported menus as models

The view exports the background extensions and templates menu models for
consumption by the pathbar. They are exported as GMenu.

However, the consumer is not supposed to modify the menu, which is what
the GMenu API is for. Also, it needs to cast to GMenuModel everytime.

To make this clearer and cleaner, cast to GMenuModel at the point when
the GMenu API is not going to be used anymore.
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