TODO:

* Search function

Partially done:

* add support for chapters
* put all callbacks / helper functions in separate files (less of those now. Is it useful?)

DONE:

* Replace gtkhtml by somthing lighter, or diminish dependencies (replaced by gtk-webcore)
* Provide #-defines for debug information
* Removing hard coded paths for help documentation, usage of a config file. 
  (only the help index file is hardcoded)
* Provide help registering (add keys to /etc/gpe-helpviewer.conf), done by gpe-helpindex app. 
* Changing all icons to GTK stock icons, or provide new icons with error checking
  and fallback on GTK stock icons.
* Single instance