Add subsection depth to HelpWriterContext

Add subsection depth to HelpWriterContext

Necessary for exporting, e.g., the selection help as rst (#679), and
cleans up the code by making more output go through HelpWriterContext
instead of directly writing to the output file.

Also, reorganize the way help topic titles are written: HelpManager is
now responsible for writing out the title for a topic.  This makes
things behave the same as with HelpWriterContext::createSubSection().

Change-Id: If7e0ee6d4c9dd313e1523fa457f64cf3c4f891f9