Don't trigger rubberband with back/foward mouse buttons

Right now the back/foward mouse buttons work, but also if you hold either
of them and move the mouse the rubberband selection is triggered.

This is the correct fix (instead of commit d0c71a14), to match the
original code before the touch events patches,  we need to check the
back/foward buttons after mouseButtonPressed is emitted and before
triggering the rubberbad selection.