Allow running katarakt --help and katarakt --version on headless systems

Christoph Erhardtrequested to merge
sicherha/katarakt:fix_help_and_version_in_headless_system into master

Do not initialise the QApplication object before the command-line arguments have been processed.

This allows distribution packagers to run e.g. katarakt --version as a minimal smoke test in their build pipeline.

Moreover, add missing .gitignore entries and fix a compiler warning.

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