fix(dataverse): implement fix for the auto-selected items in metadata UI

[jmurugan-fzj]

• Added "No Value" option to all the combo box and also to the datetime UI elements; in case when the user does not set any value, default "No Value" will be selected which prohibits the auto selection while switching between the items.
• Added a clear button to clear all the edited UI elements, especially required for clearing the combo-box and datetime elements.
• Other minor refactorings: extracted the add_delete_button and add_clear_button as utility functions for common usage.
• Adapted and corrected the tests for the changes.