Re-implement custom audio files support. (#379)

* reimplement custom audio loading for `toast` and `key_click` using `from_mp3`

* run cargo fmt

* `try_bytes_from_config`: output with `log::trace` when file cannot be opened

* rewrite `try_bytes_from_config`

* use file `read_to_end`, log categories

* edit `register_wgui_samples`

* rework audio functions for loading custom wgui sounds
oh hey, it works!

* clean up

* allow custom `startup` and `app_start` sounds
these sounds are loaded on-demand and never registered; this behaviour is unchanged

* edit `bytes_from_config` outputs

* use return value of `read_to_end` instead of `len()` for reported file size

* implement suggested changes
- remove now-unused `register_mp3_sample_from_assets` function
- replace implementation of `try_bytes_from_config`
  - missing the `real_path` context

* swap out `context`s for `inspect_err`s with closures to output `real_path`

* clean up `audio.rs`
- remove unused import
- unify capitalisation + quotes in output messages.

dash-frontend/src/frontend.rs

@@ -183,10 +183,19 @@ impl<T: 'static> Frontend<T> {
 
 	fn play_sound(&mut self, audio_system: &mut audio::AudioSystem, sound_type: SoundType) -> anyhow::Result<()> {
 		let mut assets = self.globals.assets_builtin();
-		let sample = audio::AudioSample::from_mp3(&assets.load_from_path(match sound_type {
+
+		let path = match sound_type {
 			SoundType::Startup => "sound/startup.mp3",
 			SoundType::Launch => "sound/app_start.mp3",
-		})?)?;
+		};
+
+		// try loading a custom sound; if one doesn't exist (or it failed to load), use the built-in asset
+		let sound_bytes = match audio::AudioSample::try_bytes_from_config(path) {
+			Ok(bytes) => bytes,
+			Err(_) => assets.load_from_path(path)?.into(),
+		};
+
+		let sample = audio::AudioSample::from_mp3(&*sound_bytes)?;
 		audio_system.play_sample(&sample);
 		Ok(())
 	}