Fix various issues on the Welcome dialog

[vkbo]

Summary:

This PR:

• Hides unused controls on the Create New Project page when the option to populate from example project or copy project is selected.
• Adds check that the target folder does not exist before extracting the sample project.
• Drops the Language setting from the Create New Project form.
• Replaces the QDialogButtonBox with a set of regular buttons, moves the Create New Project buttons to the same line, and adds a filter that will show/hide buttons depending on which page is active.

Related Issue(s):

Closes #1705
Closes #1706

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