Write libpkg plugin to generate placeholder .app bundle

Currently the `desktop2app` process is using a `QFileSystemWatcher` on well-known locations for desktop files to generate placeholder .app bundles for newly installed (e.g., by `pkg`) applications.

We might want to write a libpkg plugin that would call the (slightly modified) `desktop2app` script instead, removing the need for a background process.

Thanks @crees for the suggestion.

Downside: This would not cover other methods of installation, just `pkg`.

http://dnaeon.github.io/introduction-to-writing-plugins-libpkg/

Another (easier?) possibility would be to just wrap pkg, e.g, in our own `app` command.