EogPreferencesDialog: Make use of extra space when resizing dialog

https://bugzilla.gnome.org/show_bug.cgi?id=729498
EogApplication: Activate per-app plugins in the startup callback

If activated in the init function the application could not be
as initialized as expected by the plugins. Also this avoids the
unneeded plugin enumeration when reusing an instance.

https://bugzilla.gnome.org/show_bug.cgi?id=729494
Prepare for release 3.12.2
Post-release version bump