UI: Warn user if multiple instances of the UI are open

Uses a named mutex to detect if multiple instances of the program are
open, and if so warns the user.  When running in portable mode, uses a
separate unique mutex name mapped to the user's config directory to
ensure that no two portable builds use the same config directory.  This
way, portable builds do not conflict with normal builds or other
separate portable builds.

UI/obs-app.hpp

@@ -176,6 +176,7 @@ static inline int GetProfilePath(char *path, size_t size, const char *file)
 	return window->GetProfilePath(path, size, file);
 }
 
+extern bool portable_mode;
 extern bool opt_start_streaming;
 extern bool opt_start_recording;
 extern bool opt_start_replaybuffer;