Don't keep track of middle mouse press in a variable

This fixes the rendering problem in 321212. Why? Because what was happening is that for some reason the middle mouse press to gets to the PageView but not the release, this meant that the PageView got stuck into mouseMidZooming mode and all sort of bad things happened. Now since we don't cache if the mid mouse button is pressed (we just query it when needed) everything is better.

Ideally i'd like to know why we get the press and not the release, but not sure that's feasible to "fix".

BUGS: 321212
REVIEW: 111901
FIXED-IN: 4.11.0