allows the user to select the messages to be split.
is accessed with ?action=splittopics;sa=selectTopics.
uses 'select' sub template of the SplitTopics template or (for
XMLhttp) the 'split' sub template of the Xml template.
supports XMLhttp for adding/removing a message to the selection.
uses a session variable to store the selected topics.
shows two independent page indexes for both the selected and
not-selected messages (;topic=1.x;start2=y).