R1:a5b3e33e3c26

Don't show workboard action previews if the action won't have any effect

Summary:
Ref T10335. When you (for example) drag a "Resolved" task into a column with "Trigger: change status to resolved.", don't show a hint that the action will "Change status to resolved." since this isn't helpful and is somewhat confusing.

For now, the only visibility operator is "!=" since all current actions are simple field comparisons, but some actions in the future (like "add subscriber" or "remove project") might need…