FakeReader improvements, part 1

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12430 and https://trac.openmicroscopy.org.uk/ome/ticket/12423.

For 12430, running the ```mkfake``` test case from the ticket and importing into OMERO should result in a single Screen with 2 Plates being created, instead of all Images being unlinked and in datasets.

For 12423, specifying channel colors via the ```color_x``` keys (where ```x``` is the channel index) should allow each channel color to be set individually.  The colors shown in the OME-XML or upon import into OMERO should match the values set by the ```color_x``` keys.

/cc @dpwrussell, @bpindelski