system df: fix image-size calculations

Fix the image-size calculations of system-df, where the shared size is
the actual shared size with other images (including children) and the
(total) size is the sum of the shared and unique size [1].

To calculate parent/child relations, make use of the recently added
layer tree which allows for quick (and cached!) calculations.

Break calculating image disk usages into the image runtime to a) access
the layer tree, and b) make the code easier to maintain and extend.

[1] https://docs.docker.com/engine/reference/commandline/system_df/

Fixes: #7406
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

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