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/proto/WholphinDataStore.proto

@@ -36,6 +36,7 @@ enum PlayerBackend{
 
 message MpvOptions{
   bool enable_hardware_decoding = 1;
+  bool use_gpu_next = 2;
 }
 
 message PlaybackOverrides{