refactor: move async runtime to application module

To have a clear distinction between code dealing with OS process
characteristics and code of ncspot itself, it makes sense to move the
async runtime together with ncspot as it doesn't have anything to do
with the OS process.

src/mpris.rs

@@ -8,6 +8,7 @@ use tokio_stream::StreamExt;
 use zbus::zvariant::{ObjectPath, Value};
 use zbus::{dbus_interface, ConnectionBuilder};
 
+use crate::application::ASYNC_RUNTIME;
 use crate::library::Library;
 use crate::model::album::Album;
 use crate::model::episode::Episode;
@@ -19,7 +20,6 @@ use crate::queue::RepeatSetting;
 use crate::spotify::UriType;
 use crate::spotify_url::SpotifyUrl;
 use crate::traits::ListItem;
-use crate::ASYNC_RUNTIME;
 use crate::{
     events::EventManager,
     queue::Queue,