main-toolbar: Enable the gear menu only when the item is loaded
The "app.gear-menu" GAction is enabled and disabled from both photos_application_actions_update and the MainToolbar when the mode changes. This makes things unpredictable by relying on the order in which the callbacks are invoked. Currently the MainToolbar was winning and rendering parts of commit 031df27c ineffective.
Instead, this should only be handled in one place - photos_application_actions_update. It's a lot more fine-grained and informed than the MainToolbar itself. eg., the MainToolbar only knows about the mode changing to PREVIEW, but doesn't know whether the item has finished loading.
This code was added in commit 7e12154b, and the above rationale held true even then. However, back then, the item's loading state was not looked at. This is probably why the problem got overlooked.
Split out from !83 (merged)