Sets this to be activatable.
If an item is activatable, double-clicking on the item, using the Return key or calling
activate will activate the item. Activating instructs the containing view to handle
activation. `GtkListView` for example will be emitting the [[email protected]:GtkListItem:activate] signal.
By default, list items are activatable.
| this |
a `GtkListItem` |
| activatable |
if the item should be activatable |