eog-window.c: GtkAction -> GAction

This patch replaces all the GtkAction related code in eog-window.c to its
GAction equivalents.

Some things to note:

    1) I only changed the lines required to change, to keep the diff as small
       as possible. I do think some parts in eog-window.c can do with some
       cleanup and a consistent style needs to be applied. (I tried to match up
       with whatever style was used in the function I was working in.)
    2) I renamed _some_ functions and arguments to better indicate what they're
       for, but only those lines I already touched anyway.
    3) actions_window is now directly mapped onto the
       GtkApplicationWindow/EogWindow.
    4) We might want to change the action names to short, lowercase names as the
       documentation suggests.
    5) The UI is not working anymore, due to incompatibility: it requires
       GtkAction. This left me unable to test some/most of the changes, however
       I quickly added some accelerators to those actions I could and it all
       seems to be working so I have faith the other changes will to.

What's left to do:

    1) Test all changes.
    2) Remove deprecated and not-working UI code.
    3) Add accelerators to all actions.

https://bugzilla.gnome.org/show_bug.cgi?id=741050
src: remove all code related to EggEditableToolbar

This patch removes all code for the GtkToolbar inside src/. This is necessary
because the new GActions are incompatible with it and it needs to be removed
for the UI update as well.

https://bugzilla.gnome.org/show_bug.cgi?id=741050
Remove all other references to the toolbar

This patch removes all further references to the toolbar.

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

This patch removes all code for the GtkMenuBar in Eye of Gnome. This is
necessary because the new GActions are incompatible with it and it needs to be
removed for the UI update as well.

https://bugzilla.gnome.org/show_bug.cgi?id=741050
Add back eog_window_open_editor as we might still need it

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