Install dunstify

[braunbearded]

Put dunstify in $PATH, so you can run it easily without manually moving files, like dunst itself

[bebehei]

I guess you have to fix the tests 😉 You have to make changes here: https://github.com/dunst-project/dunst/blob/master/test/test-install.sh

[braunbearded]

Fixed: Dont know how I missed that

[tsipinakis]

I have rejected similar PRs before on the notion that dunstctl aka #651 would be feature-equivalent with dunstify and one could use that instead.
This doesn't look to be the case looking at the current implementation in that PR, so I agree here, let's add dunstify officially in.