Fix the logic for getting line numbers in Python 3.8

This fixes an issue in Python 3.8 where some lines couldn't be breakpointed.

Fixes #366.

Show the value of numpy scalars when using the safe stringifier

Fix errant command line expose after search

In PR #353 I added the option to hide the command line window but I
recently noticed that the window errantly appeared if you did a search.
This commit fixes that by resetting the command line window weight to 0
if it should be hidden, after a search is activated. It seems this is
necessary because URWID sometimes rebiases the widths itself and thus a
0 width may get set to a non 0 value and would be exposed.

I also fixed a bug which has always existed, even prior to my first PR
and that is if you opened a search, then did a ctrl+x to the command
window and then again back to the source window then the source window
would get focus instead of the search window. This commit fixes it so
that focus goes back to the search window, if it is still open.

There is also a small change to ui_tools.py is merely to remove a
redundant "else" that I noticed and some misc other tiny aesthetic code
improvements that I noticed in debugger.py.

This PR completely supercedes my previous PR #352 fix for these issues
because PR #352 did not correctly maintain the size of the command
window if it was re-opened.

Add option to hide command window when not in use

Also removed the redundant "jk_sidebar_scroll" option. jk scroll should
just be always enabled in sidebars like in main source window.

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.

Document the how to set new breakpoint() builtin in Python 3.7

Detect external tty size on python 2

Show callable attributes in variables view, drop Python 2.6 support (Closes gh-342)

See merge request !9

Merge branch 'show-callable-attributes' of gitlab.tiker.net:inducer/pudb into show-callable-attributes