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: E315 ml_get error when using Python and hidden buffer.
Solution: Make sure the cursor position is valid. (Ben Jackson,
closes#4153, closes#4154)
Problem: Python cannot handle function name of script-local function.
Solution: Use <SNR> instead of the special byte code. (Ozaki Kiichi, closes
#3681)
Problem: May get ml_get error when :pydo deletes lines or switches to
another buffer. (Nikolai Pavlov, issue #1421)
Solution: Check the buffer and line every time.
