Improve usability of diagram viewer
The usability of the diagram viewer can be improved:
- When zooming with ctrl and mouse wheel, zoom in on the current location of the mouse pointer (rather than zooming from the origin and shifting the content).
- Content panning using control drag would be convenient.
- The performance of the grid canvas can be improved. During panning the overhead of redrawing the whole canvas becomes noticeable.
- On Linux, zooming does not work properly: Only the grid canvas is scaled, the content remains unchanged.