Return wxNullColor rather than the original color on cancel so that the
calling code can actually tell if it was cancelled.
If one of the subs edit box color buttons is cancelled, undo the changes
made rather than restoring the original text of the active line. This
makes the cancel button actually work with multiple lines selected, and
eliminates some undo state noise.
Closes#1465.
Originally committed to SVN as r6663.
0d4846d012