Refactor dialog closing

[vkbo]

Summary:

This PR changes the dialog parent class to overload the virtual reject() slot and add a call to close() to it. This makes this a usable slot for all dialogs as this is the behaviour we want in novelWriter. It also solves an issue where close() is used as a slot, even if it isn't a valid slot.

This also fixes a bug introduced with the NDialog class in the 2.5 release cycle.

Related Issue(s):

Closes #1915

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