Extending ofxGui with new widgets

@frauzufall
awesome!
works beautifully.
yet im getting a SIGABRT when exiting.
this is thrown to the console

libc++abi.dylib: terminating with uncaught exception of type std::__1::system_error: mutex lock failed: Invalid argument

It happens in the ofxGuiGroup's destructor.
I'll check whats going on this weekend. I have no time before.

Best!