Make it possible to handle mousemove events externally on the higher level

I'm placing the map in a resizable element, but when I drag the handle and move it over the map (trying to make it smaller), the `mousemove` handler stops propagation of the events to the higher level, so the "resizable" UI element can't resize it.

I'm not sure if it was intentional, but I can't think of a reason to block all `mousemove` events.

At first I solved it by moving the `return false` statement to the `if (mouseDown) { }` block, but then I changed it so that the function is not even bound when not moving the map.