GUI: track selection highlighter

[Kinsu]

Hi Mr Spoon, thank you for contributing !

I think it's a useful GUI improvement, and I really like the idea to add an animated border. Maybe a kind of "bumping" effect could be cool, with the scale of the border being periodically increased (5-10%) and decreased (eg. with 1 second period) ?
I don't know how to achieve this to try it, however... an animated file, or directely a scale change over time of the border in Irrlicht ?