HaikuDepot: MarkupParser: Added SetStyles().

 * Also include the terminating \0 in the loop
   over the characters, in order to consume the
   last paragraph correctly when it's not finished
   by a line break.
HaikuDepot: TextDocumentView: SetInsets(), control scroll bars.

 * Added SetInsets() to allow for a margin around the text.
 * Update BScrollBars if they exist.
HaikuDepot: Use a TextDocumentView for the Changelog.

 * This allows simple media wiki markup in the changelog.
HaikuDepot: Beef up the Sequitur changelog with more markup.

 * The bullet paragraphs were already working, now we have
   bold and italic plus the heading style. The links in the
   original changelog are not yet supported.