Only update the cursor selection once per mouse-down

In Chrome, right-clicking repeatedly generates both a contextmenu AND
a mouseup event for subsequent clicks. E.g. a sequence of right clicks
generates:
1st - mousedown, contextmenu
2nd...nth - mousedown, contextmenu, mouseup

This causes problems when clicking on images as the selection is
immediately reset after being processed by the first context menu,
causing the subsequent mouseup event to try and reset the cursor
selection *again*.
Hide the cursor anchor element

This is not a visible element. It is used only to track selection
ranges inside the document.
Add some additional documentation to mouse click handling
Maintain selection when right-clicking on a range that ends at an image

On OSX, right-clicking on an image at the end of a range selection will
hit this particular branch. The image should remain selected if the
right-click occurs on top of it as technically it's the same behaviour
as right clicking on an existing text selection.
Merge pull request #483 from peitschie/right-click-behaviour

Improve right-click behaviour near and around images