Missing Settings from the Permissions Section that are present in third-party tools (Flatseal)

### Problem

There are a few tweaks to permissions like allowing access to external drives for Steam or applying environment variables to apps that can only be done via the command line or through a third-party tool called [Flatseal](https://flathub.org/apps/details/com.github.tchx84.Flatseal) from Flathub.

### Proposal

I think that a some sort of text field should be added beneath the Home/Root folder toggle to be able to specify the directory to the external drives. And for environment variables, a separate section could be added at the bottom with text fields and stuff.

### Prior Art (Optional)

#### Filesystem access

#### Settings for environment variables