UI: Grammar with non-string types

[stanislaw]

Also editing requirement with non-string grammar types:

• [ ] SingleChoice: Allow editing options in the UI using a dropdown menu.
• [ ] MultipleChoice: Allow editing options in the UI using a dropdown menu.
• [x] SingleChoice: Prevent a node/requirement form from being saved if a value is out of range.
• [x] MultipleChoice: Prevent a node/requirement form from being saved if a value is out of range.

NotImplementedError: GrammarElementFieldSingleChoice(title = "COVERAGE_SINGLE", options = [2 elements], required = False, _tx_position = 357, _tx_position_end = 440, parent = GrammarElement(...), gef_type = "SingleChoice")

This ticket can be done together with https://github.com/strictdoc-project/strictdoc/issues/1022.