Fallback to GtkApplicationWindow key_press_event() when a key press is not handled.

I have a pretty rudimentary knowledge of Gtk, so what I'm suggesting might not be the right way to fix this; if so, please let me know.

After going through the code and trying to understand what was causing #28 (closed), I came across the fact that DzlApplicationWindow overrides GtkApplicationWindow key_press_event() handler in order to integrate with DzlShortcutManager.

Since we're not just connecting to the key-pressed event but rather overriding the default behavior, I believe that if we don't handle the event, it is not enough to only return the correct value, but rather call the parent's key_press_event() and let it handle what it's supposed to handle.

GtkWindow already has the logic to handle Shift-Tab (and probably other cases as well). I don't have enough knowledge on libdazzle codebase to be sure this won't cause any side effect, but I don't think it will.