The context menu is now created the way SWT is meant to be used (setMenu() on canvas). The native way, if you will. This prevents the memory leak we had before.
To be economical with system handles, we create the popup menu items only when needed and dispose them when the menu is hidden.
Careful: the changes mentioned above cause the context menu __not__ to appear when dragging the mouse with the right button pushed. The question arises if zooming should be enabled with the right mouse button. Wouldn't it be more convenient to allow only the left mouse button to zoom?