Figured out the problem, put in an if statement

The problem is that when the mouse doesn't move, inputNum doesn't
get changed back to null. Just realized that now. Perhaps I can
do a less hacky way than the if statement.
Fixes other things, like scrolling value calculations
Hides and unhides num-manip element, not at the right time though
Since numManipulator is in scope, use it?
numManipulator in the right place, but doesn't get events
Why is the absolutely positioned elem not staying a target?
Changes visibility toggling to css

- I'm not sure this is better - this splits the behavior up into
separate files, perhaps not as clear
Adds clarifying comments, also read note about css below

- If visibilit is handled in the css, .cm-number and .num-manip
need to be related in some way, either parent and child or as
siblings or something.
num-manip can be moused over!!!