EogApplication: add accelerators to GActions

This patch adds accelerators to those GActions whose GtkActions
used to have accelerators.

https://bugzilla.gnome.org/show_bug.cgi?id=741050
EogApplication: Make accelerator assignments easier to read

Based on code found in Evince's EvApplication class.

https://bugzilla.gnome.org/show_bug.cgi?id=741050