Fix missing data path issue when creating themes folders

[vkbo]

Summary:

The folder for custom GUI and syntax themes were attempted created before the data folder for novelWriter was created. This printed four error messages the first tome novelWriter was run on a new system. The problem resolved itself on the second run.

The error was handled, and the folders in question are read-only, so the error was trivial.

In the same PR, the functions to set dataPath and confPath in the main config object have been removed. They are not in use, neither by the app nor the test suite. A new common function to both check for and create folders at the same time, has been added. This is useful also elsewhere in the code.

Related Issue(s):

Resolves #1180

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