Added register functionality (and minor changes)

[FEATURES]
- some methods changed to const
- some methods changed to virtual
- exposed internal data of the GroundSensor
- added "register sensor data" functionlality
- added "register sensor data" configuration window
- added program icon
- TrafficEngine::singleStep method implemented for real and working
- hided step button (as it's not working any more)
- hided log sidebar (as it has never be used)

[BUGFIX]
- agent moves correctly in relation to his speed
- consistency on the "show sidebars" check status and sidebars visibility
- added the QtXml library to the linking library needed to fully compile