Is your suggestion for improvement related to a problem? Please describe.

It is possible to enter a non existing relative path or absolute path in the library properties dialog

Describe the solution you'd like

There should be a check if the path is existing or not and displayed in the UI.

If it's a relative path it should be resolved against the current directory of the library (getDatabasePath() in bibdatabasebcontext
and then checked for existence
If it's an absolute path it should be checked for existence

Add a validation check to the UI (see other UI dialogs for example)