Item 7 from #33610:
Change fixed # of lines to variable # of lines as needed, up to limit.  About 15 is limit for 4-space indents in 80 char lines. 

* Change default number of lines to be 15 in config.
* Label in config dialog.
* Initialize context to be one blank line to show it's turned on.
* Remove padding of blank lines.
* Modify tests to show variable number of lines in context text instead of lines scrolling out of view.

Tip: For manual tests, help.py works really well in viewing 15 lines.