Presenting with PowerPoint or Impress triggers traceback

[Tomas Groth]

Confirmed in 2.3.2, on both Linux and Windows.

The likely cause is the QT API has changed from 4 to 5.

Traceback (most recent call last):
  File "/home/tgc/projects/OpenLP/code/trunk/openlp/core/common/openlpmixin.py", line 58, in wrapped
    raise e
  File "/home/tgc/projects/OpenLP/code/trunk/openlp/core/common/openlpmixin.py", line 54, in wrapped
    return func(*args, **kwargs)
  File "/home/tgc/projects/OpenLP/code/trunk/openlp/core/ui/slidecontroller.py", line 1142, in grab_maindisplay
    win_image = QtGui.QScreen.grabWindow(win_id, rect.x(), rect.y(), rect.width(), rect.height())
TypeError: QScreen.grabWindow(sip.voidptr, int x=0, int y=0, int width=-1, int height=-1): first argument of unbound method must have type 'QScreen'