MouseEvent: Clarify button-number and pointer-ID relation incl. case 'no button/pointer', i.e. button == 0, pointer-ID == -1

doPointerEvent:
  - allow id==-1 -> button==0 for no button, i.e. mouse move

doMouseEvent:
  - keep button 0 value, i.e. map to pointer-ID -1
Bump 7u45