PackageInstaller: Cleanup and code simplification
PackageInstaller: More cleanup and code simplifications
PackageInstaller: Yet more cleanup and code simplifications
PackageImageViewer: Re-use B_QUIT_REQUESTED
PackageInstaller: Refactoring and cleanup

 * Extraced common base class BlockingWindow from PackageTextViewer and
   PackageImagerViewer.
 * Use layouted version of PackageTextViewer.
 * Center ReadMe on screen.
 * Apply margins around the text view.
PackageTextViewer: Fix include position
PackageItem: Fixed rewriting item paths

 * The version of BString::Replace() that was used takes the offset at which
   replacing should start, not the number of replacements. So did this ever
   work? Use ReplaceFirst() instead.
 * Address the performance issues which Pawel commented on, plus some more.
 * I've left some debug output in the code (commented out), since I want to
   work on this some more. For example, I noticed that GoBe Productive puts
   files into /boot/beos/...
Add Web Open Font Format to mime_db.