Fix overzealous transactions on dashboard panels

Summary: Fixes T10474. This had the same root cause as T10024 -- a missing call to didSetValueFromStorage() because of the way the fields work.

Test Plan:

• Edited a text panel before change, without changing text: got silly transaction.
• Made change, edited text panel without changing text, no transaction.
• Made a real edit, got a good transaction.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10474

Differential Revision: https://secure.phabricator.com/D15391