* Add factory methods for ComputedList/Set that can be called
with a lambda. (ComputedValue already had such a method.)
* Add tests, also for ComputedValue.
* Update documentation and null check on ComputedValue.

Change-Id: I8e042d661a7dd14de5a8a6a7ad7fa74e689ef3ad
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/189189
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Jens Lideström <jens@lidestrom.se>