partially solve Issue31

This is a temporary solution to #31, by this the zooming is usually on right place.
Except that zoom by default.
There are some factors cause this issue:
* MAx2 MAy2 value will change after a while for unknown reason, use DX, DY as judgement
* Scale the value of scrollby is not require
Also I fix the Position label in qucs_init.cpp, that is not working, simply call printCursorPosition on MouseMoveEvent