Allow deleting modules/themes/layouts via the "Uninstall" function of Project Installer.

We made such a huge improvement when we added the Project Installer! Users now don't have to touch their sftp or webhost control panel in order to add modules, themes and/or layouts to their Drupal site. We take care of downloading them, extracting them in the appropriate folder and doing the enabling. When it's time to uninstall a project from the installation though, we stop at the disabling step. Would be great to offer the option to also completely remove from the filesystem through the UI.