Zooming reworked.

[Feature] Zoom with middle-mouse-wheel and ctrl (see request 0000125)
While zooming the region under the mouse cursor stays at its place, when using middle-mouse-wheel.
While zooming with menu or toolbar, the center of the visible region stays at its place.

[Fix] Zoom levels increase by 20% (see request 0000128)
Zooming is now more general, using a QMap instead of an KDisserPart-attribute for each zoom level.
Adding more zoomlevels can be done at one place very easily.

[Change] Zooming an item under the mouse coursor, although it is not selected is *removed*.
This behavior is completely surprising and hence it violates certain usability guidelines.