Normally, widgets will loose a button pressed state when loosing the focus. This is a normal behaviour if we have a single pointing device. The widget implementation expects it and may rely on it.  

If we need to control more widgets at the same time, the pressed state must not removed wit the focus. In this case we have no mouse emulation, but real multi touch support.

In this branch I have done both solutions. 
1.) A single point touch mouse emulation for all widgets in MixxxApplication. The touch point behaves exactly like a Mouse.
It is the same what X11 does for Qt4 and Qt5 on its own.
2.) A real multi touch solution in WWidget. It is still using Mouse events, but the widgets are prepared. This solution is not effected by the focus. 

Most widgets are working fine with 1.) only for buttons and sliders it is sensible to control simulation (multitouch) so they need 2.)
That's why WWidget is the best place for it.