Support custom mpv.conf (#327)

Part of #235 
Closes #305

Adds a preference for specifying a custom `mpv.conf`

Also adds a preference to toggle between `vo=gpu` and `vo=gpu-next`, see
https://github.com/mpv-player/mpv/wiki/GPU-Next-vs-GPU

There are some caveats:
* Multi-line text entry via a remote is rough
* [Track selection
options](https://mpv.io/manual/stable/#track-selection) may not work as
expected because Wholphin manages this itself
* Bad settings will crash the app hard

Dev note: This rewrites the text field inputs to use the [state based
implementations](https://developer.android.com/develop/ui/compose/text/migrate-state-based)

app/src/main/java/com/github/damontecres/wholphin/util/mpv/MpvPlayer.kt

@@ -57,6 +57,7 @@ import kotlin.time.Duration.Companion.seconds
 class MpvPlayer(
     private val context: Context,
     enableHardwareDecoding: Boolean,
+    useGpuNext: Boolean,
 ) : BasePlayer(),
     MPVLib.EventObserver,
     TrackSelector.InvalidationListener {
@@ -100,7 +101,7 @@ class MpvPlayer(
 
         if (enableHardwareDecoding) {
             MPVLib.setOptionString("hwdec", "mediacodec,mediacodec-copy")
-            MPVLib.setOptionString("vo", "gpu")
+            MPVLib.setOptionString("vo", if (useGpuNext) "gpu-next" else "gpu")
         } else {
             MPVLib.setOptionString("hwdec", "no")
         }