Bug 577332 - In Compare editor, Generic Editor doesn't configure viewer

...for baseline documents because the document isn't backed by a buffer
so its content-types cannot be resolved.
For the compare viewer, we store the content-types that could be
resolved for other panels and use them as fallback when no content-type
can be resolved.

Change-Id: I2425d8b826ca31c79a68cf719d7bb2903e54750f
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/187892
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Mickael Istria <mistria@redhat.com>