When the user right-clicks on an image, you might want to show a
special context menu, separate from the regular link context menu.
This patch only implements enough of the functionality to get this
working in a single-process context.
This works everywhere right now, but it's obviously not going to stay
that way forever. :^)
Note that this does not advance the cursor correctly for whitespace
since the cursor is DOM-based and doesn't take whitespace collapsing
into account yet.
