rc2 image changelog

[jpstroop]

@mikeapp I think there is only one remaining normative change to be written up:

• Please clarify case sensitive of types (and more generally for property values on which data model depends?) #1703

There are about 20 Non-normative changes to consider for inclusion

[mikeapp]

Updates to reflect #1685 and #1787, and also to call out the language map pattern for label.

#1703 references the Image API; however the related PR (#1704) doesn't update Image, only Prezi. There's no equivalent to Prezi 4.1 in Image.

[zimeon]

Part of work on #1830

[azaroth42]

As @mikeapp contributed to the PR, merging.