Associate the label with its mnemonic target.
If the label has been set so that it has a mnemonic key (using i.e. [method@Gtk.Label.set_markup_with_mnemonic], [ method@Gtk.Label.set_text_with_mnemonic], [ctor@Gtk.Label.new_with_mnemonic] or the [property@Gtk.Label:use_underline] property) the label can be associated with a widget that is the target of the mnemonic. When the label is inside a widget (like a [class@Gtk.Button] or a [ class@Gtk.Notebook] tab) it is automatically associated with the correct widget, but sometimes (i.e. when the target is a [class@Gtk.Entry] next to the label) you need to set it explicitly using this function.
The target widget will be accelerated by emitting the [signal@GtkWidget:
GtkLabel:mnemonic-activate] signal on it. The default
handler for this signal will activate the widget if there are no mnemonic collisions and toggle focus between the colliding widgets otherwise.
the target `GtkWidget`, or null to unset