Syntax highlighting improvements in the IDE

[GeorgSchneider]

1. Color types differently than keywords (currently, they're both blue for me)
2. Have a separate coloring for ledger actions (create, exercise, fetch etc.)
3. Consider do a normal keyword (it's currently colored differently than other keywords)
4. Empty list literal [] is colored blue like a keyword, in non-empty ones the brackets are white (same as in type definitions like [Party] they're also white)