accels: Sync app and menu accelerators

When porting to using gtkapplication accelerators, there was a regression
in that the in-menu accelerators were not changed/disabled accordingly, which
meant that they were still being activated even when the global accelerator
was disabled.

https://bugzilla.gnome.org/show_bug.cgi?id=728121
(cherry picked from commit 6e0bcf62bdb8b1c5e3f1fe2b7d339b674138f8c6)