PackageInstaller: Run install scripts in the correct directory

 * From debugging with the Gobe and Moho installer, scripts define which
   folder to run them from. The PackageInstaller is supposed to run the script
   in that working directory. The parser seems to have the correct folder in
   "installPath" when adding the script as PackageItem, but that code is rather
   horrible. I've changed it so PackageScript items also set the path, use
   InitPath() to obtain the final working directory and set it before running
   the script.
 * Both Moho and Gobe create the Deskbar link from that script, the folder
   is rewritten in the script via ReplaceAll().
 * Correctly running the script makes a bug visible: Dynamically added files
   in the install location by these scripts are not removed when uninstalling
   the package. When re-installing a package, it is first uninstalled and this
   currently gives an error for both Moho and Gobe, since they create some
   links in these scripts which never worked before. To install again, the
   install folder needs to be deleted manually.
 * Some cleanup along the way... sorry.