Navigator: Show position marker in all Stacks

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.
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
Navigator: add addAction function
add static data dir to urls
Navigator: some cleanup

- renamed onmousemove.move to onmousemove.drag
- whitespace
Navigator: Hide position markers onmouseout

Hide all position markers when the mouse leaves the focused stack.
Otherwise they would remain somewhere near the edge.
Navigator: Make strict