Navigator: Add position marker functionality

Shows a "Position Marker" (duplicate mouse cursor) in all open Stacks.

The code has been is copied from the Selector tool.
Navigator: Change position marker to a simple cross
Navigator: Move position marker while dragging
Navigator: Move stack only on left mouse button drag

Let the Navigator only catch the mousedown event if it comes from
a left mouse button. Before, a right click would open a context
menu but also trigger the move action.
Navigator: add addAction function

This allows for easy adding of actions (in derived tools, such as
the Tracing Tool).
Tracing Tool: inherit basic actions from navigator
Tracing Tool: change existing actions to use prototype
Navigator: Remove redundant definition of 'onmousewheel'
Navigator: Don't modify statusbar
add static data dir to urls
Navigator: Hide position markers on mouseout

When the mouse cursor leaves the stack, hide the position markers
rather than having them stuck at the edge.
Navigator: Tidy up and make strict

 - Add some ";"s

 - Add some "var"s

 - replace "new Array()" with "[]"

 - rename onmousemove.move to onmousemove.drag

 - remove onmousewheel.move
   This simplifies the onmousewheel event handler by removing the
   "move" part which is not used. At the same time, this removes the
   need to rename "zoom" (which should have been called something
   like "z_scroll").
Remove Selector tool

The "Position marker" functionality (duplicate cursors in all open Stacks)
is now in Navigator. Other than that, the Selector didn't add any features
and wasn't used anywhere.
handle ctrl-key down in navigator
tidy up key handling in project

Conflicts:

	django/applications/catmaid/static/project.js