xfce4-mime-settings: add default screenshot app
I was surprized to see the Default Apps menu didn't include an option to set a default screenshot app, that would use the PrtScr
button by default. I ended up setting the default shortcuts to my new screencap app instead, requiring a fair bit more knowledge about how my distribution works than i anticipated.
Allowing users to set a default screenscap app would facilitate much of that work, and reduce that required knowledge and time.