[QOrganizerCollection] Add SecondaryColor metadata key

Similarly to Color and Image metadata keys, this could be useful
in data visualization purpose.

Change-Id: Ibd1e59b2f2bec41052eee8cd6a95eef1d47410e7
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Michael Krasnyk <michael.krasnyk@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>