Issue 43

This enhances the zoom functionality of the Diagram (in the mainframe and spawned windows). By holding down the left mouse button the user can draw a visible transparent blue rectangle to mark the area he likes to zoom in. When releasing the left mouse button the start (x,y) and stop (x,y) positions are used to reset the range of the axes. By pressing the right mouse button the original axes range will be restored ("unzoom")

Resolves #43 (closed)