Avoid parenting mess by calling the widget directly

[rozyczko]

Description

When counting instances of plots, one needs to be careful with QtWidget parentage being updated.
The existing Manager->Plotter->PlotterWidget chain was working with Qt5 (and from what I remember was necessary to achieve correct MDI space location) but started failing with Qt6.
Getting rid of Plotter and instantiating PlotterWidget directly seems to be safer.

Fixes #2527

How Has This Been Tested?

local testing on win10

Review Checklist (please remove items if they don't apply):

• Code has been reviewed
• Functionality has been tested
• Windows installer (GH artifact) has been tested (installed and worked)
• MacOSX installer (GH artifact) has been tested (installed and worked)

[butlerpd]

can merge as soon as the two reviews are positive

[tsole0]

Changes work on MacOS and Windows on installer and in developer environment. Code is consistent with methods for ensuring child windows are created with proper high-level parents, and is compatible with opening additional windows from Plotter2DWidget.

This isn't the only place Plotter or Plotter2D is called, however, so issues stemming from window hierarchy may still arise in other parts of SasView, such as in src/sas/qtgui/Utilities/ImageViewer.py (lines 142-146):

    def addPlotter(self):
        """
        Add a new plotter to the frame
        """
        self.plotter = Plotter2D(self, quickplot=True)

where Plotter2D is still called.
Because I believe Plotter2D was kept around from pre-PySide and its functionality has been supplanted by Plotter2DWidget, I wonder if we need to keep it around at all. Removing it now though would raise a lot of other problems in dependent code (as noted above), so I believe that is better left to a separate PR.

I think this PR is good to go, especially considering this PR is important for fixing a bug on a time-sensitive project.

[nouhaag]

The code works on Ubuntu in developer environment. Indeed, the dataset disappears from Data Explorer > Data menu, with all the plots linked to it. However, after clicking on the Delete button, the following error is raised several times in the terminal.

11:13:15 - sas.qtgui.Perspectives.Fitting.FittingWidget - ERROR - Traceback (most recent call last):
  File "/home/agouzal/sasview/sasview/src/sas/qtgui/Perspectives/Corfunc/CorfuncPerspective.py", line 279, in model_changed
    self.slider.extrapolation_parameters = self.extrapolation_parmameters
  File "/home/agouzal/sasview/sasview/src/sas/qtgui/Perspectives/Corfunc/CorfuncSlider.py", line 221, in extrapolation_parameters
    if self.find_parameter_problems(params) is None:
  File "/home/agouzal/sasview/sasview/src/sas/qtgui/Perspectives/Corfunc/CorfuncSlider.py", line 84, in find_parameter_problems
    if params.data_q_min >= params.point_1:
AttributeError: 'NoneType' object has no attribute 'data_q_min'

[rozyczko]

Yes, this is a separate issue in the Corfunc, and it should be ticketed