Highlights for scrubbable numbers, partially adresses #6

I added a highlight to number-tokens in Javascript code to indicate that they are scrubbable. I didn't choose any of the options in your enhancement #6, since they didn't convince me. Instead I did the following:
- Added a hover style to number tokens, so they are highlighted on mouse-over, indicating that the user can do "something" with them.
- On Cmd/Ctrl-down while hovering over a number the mouse pointer changes to a col-resize cursor; thus, indicating that the user should drag and in which direction the dragging should be
- when the text cursor is inside of the range of a number token and Alt+Shift is pressed the same highlight is added to the focused number as the one that's used for mouse-over, again indicating that this value can now be changed (nudged). 
- The highlights stay active (even after the mouse was moved out of the number) while the number is being changed (scrubbed/nudged)


This pull-request also contains the previous two (fix for #3 and fix for #6). If you need it, I can extract the changes for those two from this branch, but it was a lot easier to just leave it in there.