Usability Improvement: Field Calculator: Expand texts to avoid misinterpretation and confusion

[tschmetzer]

• "Create a new field" can easily be misinterpreted by users as an option to create a new field wondering why it's not possible to click "OK". To overcome misleadingly interpret the "Create a new field" text as such I would suggest to extend that text to something like "Create a new field with values based on expression/function" or similar. I'm can imagine many (maybe rather not extremely experienced) users have already at least stumbled upon thinking they could just create a new field here.

• "Create virtual field" could be extended to a more meaningful text like "Create virtual field with instant calculation". This is more self-explanatory and the user might not be trapped into trying to create a new (whatever virtual) field and not being able to click "OK"

This ticket was created as a result of the discussion in #41635