UI: Add flag/file to disable built-in updater

When distributing OBS via third party platforms that have their own
update systems we want to be able to disable the OBS updater without
having to resort to having a separate build entirely.

UI/obs-app.hpp

@@ -144,6 +144,7 @@ public:
 
 	std::string GetVersionString() const;
 	bool IsPortableMode();
+	bool IsUpdaterDisabled();
 
 	const char *InputAudioSource() const;
 	const char *OutputAudioSource() const;