PackageInstaller: Cleanup PackageWindow

 * Also center main package installation window on screen.
PackageInstaller: Another round of cleanup.
PackageInstaller: Make window title properly localizeable
PackageView: Refactored a lot of repetitive code...

... to create menu items.
PackageView: Small coding-style cleanup
PackageInstaller: Converted to use layout API

 * Also auto-hide vertical scroll-bars when not needed.
 * Nicer margins everywhere
 * TODO: Install type description could collapse when empty or display
   a default message.
 * More refactoring and cleanup
PackageInstaller: Install apps to non-packaged "apps" folder.

Previously, the target folder was always unset, which meant one needed
to select a target folder manually. When running from a read-only media,
this would still be a problem (a TODO), so that instead of failing to install
the package, the user is simply prompted for a path.
The change means that together with rewriting old paths to packaged
locations to the non-packaged equivalents, just double clicking a PKG and
hitting "Install" will now work. Tested with Moho installer and
Gobe Productive.