mouse: Sync touchpad scrolling switch states correctly

The old code would not sync the state when the page was loaded. Also, due to how the code was written, it would not update the switch state if the underlying setting was changed in some other way.

Fix this by connecting the setting with the "state" of the switches. When the switches "active" property is changed, update the underlying setting and ensure the mutually exclusive switch is turned off when needed.

Fixes #398 (closed)