LMS-2225 use channel colors when displaying merged images

...systemsx/cisd/openbis/dss/etl/AbsoluteImageReference.java

.../systemsx/cisd/openbis/dss/etl/ImagingDatabaseHelper.java

...cisd/openbis/dss/etl/dataaccess/ImagingDatasetLoader.java

...ystemsx/cisd/openbis/dss/etl/dto/api/v1/ChannelColor.java

...isd/openbis/dss/etl/dto/api/v1/SimpleImageDataConfig.java

...openbis/dss/generic/server/images/ImageChannelsUtils.java

...emsx/cisd/openbis/dss/etl/ImagingChannelsCreatorTest.java

...bis/dss/generic/server/images/ImageChannelsUtilsTest.java

...nbis/dss/screening/server/DssServiceRpcScreeningTest.java