public Theme::getInstallDirectory()
Returns the directory where a theme should be installed.
If the theme is already installed, drupal_get_path() will return a valid path and we should install it there. If we're installing a new theme, we always want it to go into /themes, since that's where all the documentation recommends users install their themes, and there's no way that can conflict on a multi-site installation, since the Update manager won't let you install a new theme if it's already fou