Add support for sound backends & some miniaudio changes (#4978)

Miniaudio now runs everything in a separate audio thread - this uses boost::asio's io_context.
Our miniaudio implementation is now also much simplified - it does not use its own resource manager or device. This might end up being stupid if sounds don't work after changing output device or locking or w/e
I've made the sound controller into an interface, meaning we can support multiple sound backends in Chatterino. I've added a Null sound backend that disables all sound. A QMediaPlayer or QSoundEffect or Qt backend could be added.
Miniaudio might idle & disable the device now too, not sure

I've added some unrelated changes in the form of a new setting type, and a new setting page helper function for it, which will hopefully make adding new enum settings easier in the future.
This setting stores its value as a string instead of an int, and uses magic_enum to convert between that string value and its enum value.

mocks/include/mocks/EmptyApplication.hpp

@@ -85,6 +85,12 @@ public:
         return nullptr;
     }
 
+    ISoundController *getSound() override
+    {
+        assert(!"getSound was called without being initialized");
+        return nullptr;
+    }
+
     ITwitchLiveController *getTwitchLiveController() override
     {
         return nullptr;