Drop extra packages and note which no longer applies.

* now that haikuporter resolves the dependencies correctly, there's no
  need to add texi2html and htmldoc explicitly
Refactor definition of Haiku image contents.

* Move actual definitions of respective image contents to files
  underneath build/jam/images/definitions (minimum, bootstrap, regular).
* HaikuImage now only includes the image definitions for the selected
  build profile and adds the Haiku packages.
* Simplified topmost Jamfile to some extent.
* Move definition of DESKBAR_APPLICATIONS and DESKBAR_DESKTOP_APPLETS
  from build/jam/packages/Haiku to the respective image definition file.
* Sort and reformat several lists to make them more readable.
Add files missing in last commit, sorry.