Implements Urgency Hint handling

Makes the panel button blink when the urgency flag is set.
Mode info: http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.2.4
Avoid duplicated ElidedButtonStyle instances to save some resources.
Update the ElidedButtonStyle proxy style when its base style is changed.
Return 0 for QStyle::SH_MenuBar_AltKeyNavigation style hint of mainmenu.
With this, the menu won't be closed when the user press Alt key.
Remove repeated find_package() from CMakeLists.txt

Fixes lxde/lxqt-panel#63