Add a variant on parenthetical_split() to helper.lua and use it to fix bug #20401.

If no problems are found with this approach, I can backport it to stable.

(Browse SVN revision 56023)

[remove_unit_overlay]image= does not work on images that have been affected by image path functions — ones that use commas, at least.

So, as byspel, if I have [unit_overlay] place "misc/blank-72x72.png~BLIT("units/vampires/noble.png~TC(1,magenta)~SCALE(38,38)",34,0)" on one of my units, I can't later have [remove_unit_overlay]image= remove it. I was told the way the tag's code handles commas has something to do with it.

To easily see this bug in action, do the following:

1. Get the Era of Chaos (svn cp https://wesnoth-umc-dev.svn.sourceforge.net/svnroot/wesnoth-umc-dev/trunk/Era_of_Chaos or http://wesnoth-umc-dev.svn.sourceforge.net/viewvc/wesnoth-umc-dev/trunk/Era_of_Chaos/?view=tar)

2. Start a game; play as vampires

3. Right-click on your leader and hit "Transform This Unit"

4. Transform your leader again

After four, the overlay in the upper-right corner should theoretically be removed, but it isn't removed.

The code for this ability is in utils/abilities.cfg.