Increase minimum Qt version to 5.10

[vkbo]

Summary:

This PR increases the minimum required Qt version to 5.10. This is still quite an old version (released 2017-11-30), but there is only one known place in novelWriter where a higher version is required, which is for the RegEx search to work properly with Unicode.

Technically, increasing to 5.9 would have been enough to avoid issues that reduces the functionality of novelWriter (older versions do not support non-breaking spaces the way novelWriter uses the document editor). However, with 5.10, a lot of version checks in the code can be omitted.

Ubuntu 18.04 comes with Qt 5.9.5, but from novelWriter 2.0 this Ubuntu release is not supported anyway since support for Python 3.6 has been dropped.

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