Tweak the HTML Help output to enable Previous/Next navigation in the generated

Tweak the HTML Help output to enable Previous/Next navigation in the generated
CHM, and added the toolbar buttons for this.

Also rationalised the other options a bit:
 - added Forward button (since we have Back);
 - removed Locate/Sync (since we have auto-sync enabled for the ToC);
 - removed Stop and Refresh (mostly a waste of space for help, but still
   available from the Options menu if users really want them);
 - enabled advanced facilities on the Search tab (search within results, etc).

All this seems to have had no obvious ill effects (tested on Win98), although
the resulting CHM file is slightly bigger (~3kbyte, <1%).

git-svn-id: svn://svn.tartarus.org/sgt/halibut@7176 cda61777-01e9-0310-a592-d414129be87e