Fix InputEngine::inputModes notification

The inputModes property was not notified properly when updated.

This change adds value cache and update function for the property.

Currently this property is not tracked by anything, so the bug
was not noticed.

Change-Id: Ic9e562c6e6c8f508e56f115981a52314bef05535
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>