actions: Introduce GtkAction for each keybinding.

For all existing keybindgs a GtkAction is generated with a (for now)
empty callback. All of this is autogenerated.

From now on, keybindings can be ported over to GtkAction one by one.
actions: ui_utils: Load GtkActions via GtkBuilder.

With the GtkActions being specified in Glade they have to be loaded by
GtkBuilder. A workaround for a Gtk bug (fixed in 3.6 and newer) is needed to
make default accelerators work.
actions: Convert File->New to GtkAction
actions: Convert File->Open to GtkAction
actions: Convert File->Open Selected File to GtkAction.