Move GuiDisplay Class to OnlineMonitor

Yet another class that is in object to be stored, hence should not live in src/objects. Furthermore, it now inherits directly from TGMainFrame instead of possessing one, removing the nasty macro loaded in the beginning

(see e.g. https://root-forum.cern.ch/t/error-in-tqobject-checkconnectargs-slot-xxx-does-not-exist/20098 for reference)

This class still has a lot to optimize and clean up, but I'm not sure this needs to be done now.