Send Notification when Command finished, similar to / same as the new gnome console (formerly known as kgx)

- [x] Be sure to check the existing issues (both open and closed!), and make sure you are running the latest version of Guake.

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i think the FeatHub Domain may have been sold to someone else?
it looks like a website advocating gambling now

anyways, my idea:
i would like Guake to send a notification when it has finished a command / command-chain, similar to, or even the same as the new gnome console (ehem. kgx).

this would be especially useful for guake, as guake is often "collapsed" (at least on my system).

love the project by the way, i use guake practically exclusively as the only terminal emulator on my Laptop.