Add required option to Labels tag

**Is your feature request related to a problem? Please describe.**
I would like to enforce annotators to always highlight a region in the text using one of the labels in my tag. This is not possible at the moment, because this option is not available for the Labels tag.

**Describe the solution you'd like**
I would like to see the attribute "required" added to the labels tag, which checks whether the annotator selected at least one region in the text, and otherwise gives a popup when the annotator clicks on "submit".

**Describe alternatives you've considered**
I am now either accepting that my data does not get labelled, or need to add a complex set of rules across multiple Views, Choices, and Labels tags. (e.g., a View with visible when "no-region-selected", containing a required choice that adds a control question. However, users can still ignore this reminder.)