I have not been playing with Qt5 much, what I have to say is that the plugin concept of having the dll (libs) in saperate folder was there in Qt4 also.

In Qt5 modules have changed, like QtGui has forked to QtGui & QtWidgets, and some feature in are made add-ons and not part of essential modules. I guess this restructuring of the mudules might have resulted in these new dll files. (which earlier were in QtGui4.dll / QtCore4.dll)