AUI-1300 Changing css so the scrollbar will only show up when necessary
AUI-1455 Change listener method names on ImageViewer modules

Listeners should each have a separate method, even if that method will just call a single function, as that later helps people to extend our modules. The image viewer modules had lots of listeners that are not following this pattern, so this is fixing those.
AUI-1528 Drop Zone on Form-Builder Styling is off
AUI-1481 aui-event HISTORY.md

Subtask of AUI-1458: Update HISTORY.md to reflect source code changes.
Titles for each entry are from the corresponding jira ticket.