Update Appdata XML and add screenshot

[vkbo]

Summary:

This PR updates the appdata.xml file used for AppImages, and adds a screenshot to the repo rather than link to the website where it may change more often.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed