Support controlling visibility of requirement's text

[marek-piirikivi]

Is your feature request related to a problem?

Currently text is always visible. This takes up too much space on diagrams where the focus is just on the requirement relationship.

Describe the solution you'd like

I would like to have a switch "Show text" on requirement property page.

Describe alternatives you've considered

I couldn't come up with any other alternative other than to just ignore the problem.

[amolenaar]

Fixed by #3249.