Make the flatpak(-spawn) portal nicer to work with

[orowith2os]

The flatpak module currently doesn't seem very nice to work with; ideally it would more closely look like the flatpak-spawn command, and (imo) wouldn't look as "ugly".

Here's a quick draft of what the ideal struct for it would be like, this 100% needs tweaking in order to work with the Flatpak portal, and work "properly":

use std::os::fd::RawFd;
use std::fs::File;

#[derive(Debug)]
struct EnvVar {
    var: String,
    val: String,
}

/// A (more or less) 1:1 copy of the flatpak-spawn command, as a struct
#[derive(Default, Debug)]
struct FlatpakSpawn {
    /// Enable debug output
    verbose: bool,
    /// Forward file descriptor(s)
    forward_fd: Vec<RawFd>,
    /// Run with clean environment
    clear_env: bool,
    /// Make the spawned command exit if we do
    watch_bus: bool,
    /// Expose sandbox PIDs in calling sandbox
    expose_pids: bool,
    /// Use the same PID namespace as calling sandbox
    share_pids: bool,
    /// Set environment variable(s)
    env: Vec<EnvVar>,
    // Unset environment variable(s)
    // Remember to only pass through the variable name, not the contents!
    // TODO
    // unset-env: Vec<String>

    // TODO: env-fd

    /// Run with the latest vesion of the app and runtime
    latest_version: bool,
    /// Run sandboxed, without inheriting permissions.
    /// See the --sandbox argument in https://man7.org/linux/man-pages/man1/flatpak-run.1.html 
    /// for more information.
    sandbox: bool,
    /// Whether to run with network access
    no_network: bool,
    /// Expose read-write access to the files/directories
    sandbox-expose-rw: Vec<File>,
    /// Expose read-only access to the files/directories
    sandbox-expose-ro: Vec<File>,

    // TODO: sandbox-flag
    
    // NOT PLANNED: host (use a separate struct, ideally closer to Command)
    
    /// Working directory in which to run the command
    directory: File,

    // Replace runtime's /app with DIR, or empty
    // TODO
    // app-path

    /// Replace runtime's /usr with DIR
    usr-path: File,
}

[bilelmoussaoui]

PRs are welcome.