Can install_twisted_reactor() outside of App.build
Deeper uninstall of Twisted Reactor
Allow cycles of install/uninstall of Twisted Reactor
Oops: forgot to be public about the new entry
Better docs on uninstall_twisted_reactor()
Clean unused import
Updated versionadded:: to 1.8.1
Added save_image method to Widget
Revert "Added index method to InstructionGroup"

This reverts commit cc9af2bb191338f31225c192df9739a28e29f197.
Changed save_image -> export_to_png plus bugfixes
- promoted exit_on_escape functionality to BaseWindow and implemented a
  callback for it
- PEP8 fixes
prelim focus behavior.
Add docs, provide link function, add overwrite in autopopulate, fixup.
Add focus behavior example to examples.
Add to de-focus on escape.
Remove autopopulate, instead walk the tree dynamically to find next/previous focus if not specified in focus_next/previous.
Update the example to reflect changes.
- expanded exit_on_escape callback to real function; added guard for when
  documentation is being built.
Add forward and backward iterators to iterate the widget tree.