general: Make most settings a BasicSetting

Creates a new BasicSettings class in common/settings, and forces setting
a default and label for each setting that uses it in common/settings.
Moves defaults and labels from both frontends into common settings.
Creates a helper function in each frontend to facillitate reading the
settings now with the new default and label properties.

Settings::Setting is also now a subclass of Settings::BasicSetting. Also
adds documentation for both Setting and BasicSetting.

src/audio_core/audio_out.cpp

@@ -30,7 +30,8 @@ StreamPtr AudioOut::OpenStream(Core::Timing::CoreTiming& core_timing, u32 sample
                                u32 num_channels, std::string&& name,
                                Stream::ReleaseCallback&& release_callback) {
     if (!sink) {
-        sink = CreateSinkFromID(Settings::values.sink_id, Settings::values.audio_device_id);
+        sink = CreateSinkFromID(Settings::values.sink_id.GetValue(),
+                                Settings::values.audio_device_id.GetValue());
     }
 
     return std::make_shared<Stream>(