Project bundles should be a separate File menu action

[Spekular]

As discussed in #5735, saving as a bundle should be accessed via a separate item in the File menu. This makes more sense given the current behavior of the feature (saved bundles aren't updated when the file is saved normally after exporting a bundle). It's worth considering if this feature should use "export" in the name, to make it's behavior extra clear.

[Rossmaxx]

Re milestoning to reduce scope for 1.3.