Add dark mode configuration setting in UI tab

- Allows to choose "Auto", "Always On" or "Always Off".
- If Auto is chosen, value is retrieved from OS
  - On Windows, the application needs a restart to apply the settings
  - On Windows, "Always On" is not available, the OS is responsible for
    selecting the dark palette

src/common/settings.cpp

@@ -67,6 +67,7 @@ SWITCHABLE(u8, true);
 // Used in UISettings
 // TODO see if we can move this to uisettings.cpp
 SWITCHABLE(ConfirmStop, true);
+SWITCHABLE(DarkModeState, true);
 
 #undef SETTING
 #undef SWITCHABLE