Rodin keyboard converter bug

[cfsnook]

When converting plain text to Rodin keyboard symbols using the contextual helper, it puts "invariant" keyword at the end of the preceding line

[tshoang]

The issue is that currently, we tried to support both blocks of invariants and individual invariants. When we modified the semantic model (e.g., via quick fix), the context return for invariants is individual invariants, hence the described problem. There are two possible fixes for this.

1. Drop the support for blocks of invariants
2. Distinguish between block invariants and individual invariants, i.e., using different EMF types and using the translation to normal invariants during serialisation.