Add command to update all outdated plugins

Checks for outdated plugins and updates them sequentially. I am under the impression that a markup refresh for the Plugin Manager is due relatively soon, so I have not added any GUI as requested in #1275.

Before merging, two observations.
- For user notifications, is there an elegant way to only act upon the last callback from `discover-deps`, or would storing the count in an atom be an acceptable solution?
- Currently, the functions for installing and updating plugins are called directly from the relative GUI elements; the same holds for the command added in this patch. It might be desirable to use behaviors instead.