Prevent screen turning off during playback

Closes: #157

XDG portals expose a D-Bus interface allowing apps to prevent screen
idling, user switching, and other actions. That interface,
org.freedesktop.portal.Inhibit, does all of the heavy lifting here.

Idling = the screen dimming or shutting off.

Idling is inhibited when a video is actively playing.
Idling is NOT inhibited when videos aren't playing - this includes
paused or stopped videos.

src/mpris.rs

@@ -3,12 +3,12 @@ use cosmic::iced::{
     subscription::{self, Subscription},
 };
 use mpris_server::{
-    zbus::{fdo, Result},
     LoopStatus, Metadata, PlaybackRate, PlaybackStatus, PlayerInterface, Property, RootInterface,
     Server, Signal, Time, TrackId, Volume,
+    zbus::{Result, fdo},
 };
 use std::{any::TypeId, future, process};
-use tokio::sync::{mpsc, Mutex};
+use tokio::sync::{Mutex, mpsc};
 
 use crate::{Message, MprisEvent, MprisMeta, MprisState};