PR: Fix several IPdb problems and add some improvements to it

[ccordoba12]

Fixes #4421
Fixes #4403
Fixes #4214
Fixes #4013
Fixes #3945
Fixes #3771
Fixes #3711
Fixes #3555
Fixes #1977

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This PR also greatly improves the experience of debugging in the IPython console with these new features:

• Plotting now works with a %plot magic, but only using the inline backend
  (e.g. %plot plt.plot(range(10))).
• History browsing with Up/Down arrows.
• Make the Clear console and Reset keyboard shortcuts to work.
• Show plots from the Variable Explorer.
• Change the current working directory using the Working Directory toolbar.
• Use Ctrl+Shift+C to copy text.
• Avoid blocking/interrupting ipdb as much as possible.

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TODO:

• Add tests
  • Test changing cwd
  • Test that c and n update the Variable Explorer
  • Tests for unicode variables
  • Test for %varexp
  • Test that Stop exits debugging
  • Test that variable types can't be changed in the Variable Explorer
  • Test for browsing history
• Prevent saving and loading data in the Variable Explorer while debugging.
• Fix plotting variables while debugging from the Variable Explorer.
• Show warning when trying to use Ctrl+C while debugging
• Make plotting to work
• Try to use publish_data for Pdb state
• Avoid blocking Pdb when running commands for the Help plugin
• Move automatically to first breakpoint again
• Make Up and Down buttons to browse history while debugging
• Create a mixin for browsing history while debugging (I think this is simpler and cleaner).

[ccordoba12]

This huge PR is (finally!!) over.