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).
• Equivalent to SplitTopics3 in 1.0.x