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-1480 aui-dropdown HISTORY.md

Subtask of AUI-1458: Update HISTORY.md to reflect source code changes.
Titles for each entry are from the corresponding jira ticket.
aui-dropdown did not have commit ac999be3361683ccb7a7062b5e347a5e3199dc38
which is what the other HISTORY.md files are starting with. Instead
there were two jira tickets for the commits made to this component,
so those are recorded