Problem: Heredoc for interfaces does not support "trim".
Solution: Update the script heredoc support to be same as the :let command.
(Yegappan Lakshmanan, closes#5916)
Problem: Some balloon tests don't run when they can.
Solution: Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
closes#4538) Change the feature check into a command for
consistency.
Problem: It is not so easy to write a script that works with both Python 2
and Python 3, even when the Python code works with both.
Solution: Add 'pyxversion', :pyx, etc. (Marc Weber, Ken Takata)
