Show magnifier-glass cursor over preview diagram

[oliver]

It would be nice if the mouse cursor would change to a magnifier glass if it is in the preview area, to make it clear that left-clicking will zoom into the diagram.

I keep using the wrong mouse buttons while navigating the diagram (I keep clicking left when I should right-click); and maybe that would not happen so easily if the cursor already made it clear: "if you click now, you will zoom into the diagram". So this might be a simple change which makes the previewer easier to use.