Implements Urgency Hint handling.

First of all, this dependes on lxde/liblxqt#11. As asked in #41, this PR intends to implement Urgency Hint for the taskbar. The taskbar button blinks when the urgency flag for it's window is set.
More info: http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.2.4

There may be a more lightweight solution than using QPropertyAnimation for the blinking animation, please comment. Also, how can I get a better color for the blinking, maybe based on the current theme?

@willbprog127 could you test it, if you have time?

Solves #41.