Shuttercount Frontend - Part 2 of Part 1 of 2

[JackEAllen]

Issue

Closes #2094

Description

Add a description of the changes made.
Create basic front end to allow a user to load in ShutterCount files alongside a sample.
The GUI interface design for loading ShuttterCount files is temporary and will be reworked in the future to bring related files closer together.

Testing

Describe the tests that were used to verify your changes.

• Try Loading ShutterCount files into mantid Imaging
• Loading in an unexpected format such as a spectra file triggers error dialog window on selection
• GUI tests pass
• Tests Pass

Acceptance Criteria

How should the reviewer test your changes?

• All tests pass
• ShutterCount files can be loaded into Mantid Imaging from the Loading Dialog
• Loading in an unexpected format such as a spectra file triggers error dialog window on selection
• ShutterCount Files can be loaded for a given stack after loading sample File > Load ShutterCount file for stack...

Documentation

How have you changed the documentation to reflect your changes? All changes should be noted in the appropriate file in docs/release_notes

Release Notes: docs/release_notes/next/feature-2094-Load_ShutterCount_files_front-end

[coveralls]

coverage: 72.735% (+0.005%) from 72.73%
when pulling 1a8df3d on shuttercount_frontend
into 2a6191e on main.