Add more validation checks

[alexbruy]

Add more validation checks:

• using relative path in the Attachment widget
• Attachment widget does not use local path, e.g. /home/user/some/path
• expression-based path should not be set as a default path (to work correctly it should be set as data-defined)
• database structure was changed. For now only simple check that there are no new/removed/renamed fields

Fixes #254 and #289.

ref CU-27pvwte

[ghost]

Task linked: CU-27pvwte Plugin Validations

[PeterPetrik]

Happy with changes! can you please add automatic tests?

[alexbruy]

Tests added and I tried to add them to the workflow, but I have never worked with GitHub actions before. Seems I'm doing something wrong when mounting volume to docker image and as a result test runner can't find tests.

[PeterPetrik]

👏