Allow to move only callout handle with shift key (cp#1000084)

When line callout was used, you couldn't move the point next to
the rectangle.
Now it's possible with shift key pressed.

Change-Id: I70565e4e3f80daf0e1007031ef7d49036fb0e26b
Clean up usage of 'this' and constness.

Change-Id: I35085bebdec47598a5f927924b429b02b44c0e65