Port Didcot to canterbury platform

Didcot calls Canterbury application manager to get information about
supported mime types. Remove this mechanism and start using
libcanterbury-platform API to gather information about entry points.

Apertis: https://phabricator.apertis.org/T3448



Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5621