Maya: fix fps validation popup

[iLLiCiTiT]

Brief description

Validation of fps in maya will crash if does not match because can't import popup window.

Changes

• fix popup import and add condition to show popup only if not in headless mode

[m-u-r-p-h-y]

there is another error popping up now:

# Warning: avalon.pipeline : Traceback (most recent call last):
  File "D:\REPO\OpenPype\repos\avalon-core\avalon\pipeline.py", line 631, in emit
    callback(*args)
  File "D:\REPO\OpenPype\openpype\hosts\maya\api\pipeline.py", line 503, in on_open
    lib.validate_fps()
  File "D:\REPO\OpenPype\openpype\hosts\maya\api\lib.py", line 2449, in validate_fps
    dialog = popup.Popup2(parent=parent)
  File "D:\REPO\OpenPype\openpype\widgets\popup.py", line 112, in __init__
    Popup.__init__(self, parent=parent, *args, **kwargs)
  File "D:\REPO\OpenPype\openpype\widgets\popup.py", line 38, in __init__
    geometry = self.calculate_window_geometry()
  File "D:\REPO\OpenPype\openpype\widgets\popup.py", line 137, in calculate_window_geometry
    screen = app.desktop().screenNumber(parent_widget)
AttributeError: 'PySide2.QtCore.QCoreApplication' object has no attribute 'desktop'
 # 

[iLLiCiTiT]

there is another error popping up now:

Fixed

[m-u-r-p-h-y]

it works now, fps dialogue (in case of wrong fps set) is popping up