Improve help and add jump to breakpoint

This change:

1. Some keys are re-arranged in the help pages, some blank lines
   removed, and the subtitle text is moved to the title to prune back
   the superflous words a little. Users just want to see the keys list!

2. Pressing "?" in any of the sidebar lists now displays the help for
   the sidebar keys at the top, i.e. so you can immediately see the keys
   appropriate to where you are without having to scroll down. New users
   use "?" frequently and it is a pain to have to scroll around.

3. Previously, pressing "enter" on a breakpoint list entry would pop up
   the edit page. I have added a new function so now it "jumps" to that
   breakpoint and displays that line in the source window which is what
   most users would want. You can still press "e" to edit the breakpoint
   popup. "e" was previously to toggle the breakpoint state so I have
   assigned "b" to do that, i.e. exactly consistent with "b" in the main
   source window which is what users expect.

4. Consistent with above, I have added "e" to edit the variable popup
   window, so "e" means edit in both cases.