Item12869: Some basic restructuring of SpreadSheetPlugin

 - Remove the if / elsif ... laddar,  replace it with a hash lookup
 - Group subroutines by related area.  May be useful to pull them out
   into separate loadable modules.
 - Document TWiki functions that we have not added yet.

Add unit tests for a couple of missing cases
 - EXEC and NOEXEC coverage
 - IF coverage
 - SETIFEMPTY coverage

 This also fixes a minor restriction in SUBSTRING.  Previously it used
 split on comma, which meant the string could not contain any commas.
 Changed to a greedy regex to capture the string.

 Updated unit test for SUBSTRING

git-svn-id: http://svn.foswiki.org/trunk@17621 0b4bb1d4-4e5a-0410-9cc4-b2b747904278