Fix 'bit depth' metadata of 'Picture' document

bit depth metadata extracted from image files using ImageMagick's identify command is actual the bit depth per channel.
It is needed to put in place the logic that would compute the actual image's bit depth as the metadata by itself does not exist in the image file.
It is confusing for users to see that the bit depth displayed by the Nuxeo UIs is 8 when they expect (and should be) 24.

The following identity command extracts the bit depth per channel from a 24-bit image:

identify -define registry:temporary-path=/tmp -quiet -ping -format '%m %w %h %z %[colorspace]' /tmp/24bit_image.tif[0]

Result is

TIFF 3000 2400 8 sRGB

Here is a link discussing the topic: https://www.imagemagick.org/discourse-server/viewtopic.php?t=13968

Additional note from reporter: changing the label to bit depth per channel in the UIs is an acceptable solution.