Update welcome dialog

[vkbo]

Summary:

This PR makes a few tweaks to the layout of the Welcome Dialog, and updates the illustration to the latest version.

In addition, the width parameter for the custom switch widget is removed, since it is always twice the height and therefore redundant when height is provided.

Related Issue(s):

Closes #1506

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