Automatically Managed Hotkeys

So the deal is that the registry should take over the management of the hotkey lifecycle. That way one can not have "unmanaged" hotkeys.

= NEW/MODIFIED METHODS =
Hotkeys are created or looked up with:

/* Lookup or create with default sig. if necessary*/
gtk_hotkey_registry_get_hotkey(const gchar *app_id,
                                                      const gchar *key_id,
                                                      const gchar *default_signature,
                                                      GError *error);

/* Lookup or return NULL if non-existing */
gtk_hotkey_registry_lookup_hotkey(const gchar *app_id,
                                                           const gchar *key_id);

Hotkeys are no longer deleted directly from the registry, but are deleted with a new method on the Hotkey class:

gtk_hotkey_delete();

We need a new signal GtkHotkey::deleted as well. After this signal has been emitted all ops on the hotkey should fail.

/* Set the signature, rebinding the key of already bound */
gtk_hotkey_set_signature(GtkHotkey *hotkey, const gchar *sign, GError *error);

= REMOVED METHODS =
The method gtk_hotkey_registry_delete_hotkey() will be removed from the API. In this fully managed setup we can also remove the method gtk_hotkey_registry_store_hotkey() because storing will be handled under the hood by the registry when setting properties on the hotkey.

As it says in the spec title we will also remove the gtk_hotkey_info_new() method.