Improve dialog memory handling

[vkbo]

Summary:

This PR:

• Tackles some potential issues with deleting Qt objects. It changes the split and merge dialog classes to use class methods instead, similar to other pop-up dialogs. Then it can call the deleteLater function for the C++ side after all data is processed and collected, as opposed to before when it was called on close. I am a little uncertain exactly when the delete would happen in the latter case, but I haven't had any bug reports on crashes when using these tools. Still, I think this implementation is safer.
• Adds a custom NDialog class to serve as an in-between class for QDialog. Currently, all it adds is an overload of keyPressEvent to capture Escape key presses and redirect them to close(). The default behaviour of Qt hides the dialog without going through close, thus bypassing the closeEvent logic.
• Replaces all QDialog.deleteLater() calls with a new NDialog.softDelete() method that simply sets self.setParent(None). This seems to play better together with the Python garbage collector. The Qt object does not seem to be deleted before it is released by the Python side.
• Remove deleteLater() calls on QMessageBox objects. They seem to be deleted on close already.

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