When creating shortcuts, there almost always are a trigger and an action
available for use. So make gtk_shortcut_new() take those as arguments.
Also add gtk_shortcut_new_with_arguments() so people can easily pass
those in, too.
Similar to GtkShortcutTrigger, GtkShortCutAction provides all the
different ways to activate a shortcut.
So far, these different ways are supported:
- do nothing
- Call a user-provided callback
- Call gtk_widget_activate()
- Call gtk_widget_mnemonic_activate()
- Emit an action signal
- Activate an action from the widget's action muxer
- Activate a GAction
