Set style in before compiling ui file for consistent sizing

[MikeSullivan7]

Issue

Close #2167.

Description

In gui/mvp_base/view.py the Qapplication instance style is now set before the .ui file is compiled, which avoids the Spectrum Viewer from unreliably showing in differing sizes, which caused issues with reliable eyes tests.

Acceptance Criteria

As in the issue #2167, check open MI with data and open and close the spectrum viewer a large number of times to check that it is consistently being produced at the same size which should be around QSize(914, 978) but may differ depending on your resolution. You can use python -m mantidimaging --path '/data/imaging/IMAT00010675_128/Tomo' -sv

Alternatively, you can run the screenshot tests with the added option -k test_spectrum_viewer_add_new_roi

You may want to use a print statements like

class BaseMainWindowView(QMainWindow):

    def __init__(self, parent, ui_file=None):
        self._t0 = time.monotonic()
        super().__init__(parent)

        self._has_shown = False

        print(f"{ui_file}: self.size() = {self.size()}")
        QApplication.instance().setStyle(settings.value('theme_selection'))
        print(f"{ui_file}: self.size() = {self.size()}")

        if ui_file is not None:
            compile_ui(ui_file, self)

and in spectrum_viewer\view.py:

    def show(self):
        print(f"SVWV show start    {self.size()=}")
        super().show()
        self.activateWindow()
        print(f"SVWV show end      {self.size()=}")

to see the sizing in the command line.

to check the sizing

[coveralls]

coverage: 73.378%. remained the same
when pulling a0b18bf on 2167_consistent_spectrum_viewer_sizing
into 6cec9c7 on main.

[samtygier-stfc]

Thanks. This resolves the issue for me locally.