Snap

Add APIs to override decoration managers and customize decorations #4912

Job	Run time
arches	1s
mir-libs (amd64)	17m 27s
mir-libs (armhf)	23m 42s
mir-libs (arm64)	26m 51s
snap (canonical/mir-test-tools, amd64)	12m 42s
snap (canonical/mir-test-tools, armhf)	23m 54s
snap (canonical/mir-test-tools, arm64)	21m 13s
snap (canonical/confined-shell-wip, amd64)	9m 11s
snap (canonical/confined-shell-wip, armhf)	14m 16s
snap (canonical/confined-shell-wip, arm64)	12m 46s
snap (canonical/ubuntu-frame, amd64)	7m 47s
snap (canonical/ubuntu-frame, armhf)	16m 11s
snap (canonical/ubuntu-frame, arm64)	13m 49s
snap (Miriway/Miriway, amd64)	8m 33s
snap (Miriway/Miriway, armhf)	14m 19s
snap (Miriway/Miriway, arm64)	11m 56s
	3h 54m 38s

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