Auto-completion for "Covers" section

[redcatbear]

Situation

With #23 (PR #22) we added live templates to the OFT IntelliJ plugin. What is still missing is convenience for adding requirement tags in the "Covers" section.

We already have the declared specification IDs as symbols in the IntelliJ index, so we should use that for auto-completion.

Acceptance Criteria

1. When the user fills a specification item ID in the "Covers:" section, the auto-completion shows suggestions for existing specification item IDs from the symbol index
2. The auto-completion uses search-as-you-type with a closest match against the following things with descending priority:
   2.1 Exact prefix match
   2.2. Prefix match against the name part of the requirement ID
   2.3 Sub-string match against the name part of the requirement ID
   2.4 Prefix match against the artifact type
3. (optional, only if simple to implement) Search-as-you-type auto-complete shows a short list of next best matches near the input point