Using `nice` from Tilix does nothing

[robobenklein]

For terminals in focus, all their descendants acquire the same priority values when changed, however this makes using nice for a command completely useless.

Two commands being tested: stress -c 8 and nice stress -c 8 both end up with foreground priority/niceness -5, while obviously the niced command should not be given priority.

Demo, foreground window name in top left:

https://user-images.githubusercontent.com/5423266/172304053-0bc8cf0f-5c26-4547-ab3e-4af214285680.mp4

[mmstick]

Try out #60

[robobenklein]

With #60 nice stress starts out with nice value 5, but changing focus away and then back to the terminal causes it to become -5 again. (Also the autogroup / process group niceness does not change at all)