[Lazarus] macOS trackpad horz scroll

[Alexey]

  I wanted this change:
- Carbon WS can read horz mouse wheel. it sends e.g. LM_WHEELSCROLL_HORZ
- LCL generates as usual OnMouseWheel (....Up/Down), but if it is horiz 
wheel move, then
it adds flag ssScrollHorz to Shift parameter.
- app can detect horiz wheel move, by ssScrollHorz

No need to add new events OnMouseWheelHorz, only add item to Shift.
What do you think.
Alexey