Allow setting permissions denied in global

[ghost]

There is no way to deny permissions using switches in gui, which is quite undesirable for making modifications in global. I suggest setting a 3-state switch in global.

Also, mention that changes made by manually editing the file can be deleted when updating it with gui.

[tchx84]

To add some context here, read https://blogs.gnome.org/tchx84/2022/05/31/flatseal-1-8-0/ TLDR: This requires a big re-factor on how permissions work.