Emitted when the user initiates a value change.
This is a [keybinding signal](class.SignalAction.html).
Applications should not connect to it, but may emit it with emit_by_name if they need to control the cursor programmatically.
The default bindings for this signal are Up/Down and PageUp/PageDown.
| scroll |
a `GtkScrollType` to specify the speed and amount of change |