Snap

Add APIs to override decoration managers and customize decorations (Take 2) #4977

Job	Run time
arches	1s
mir-libs (amd64)	17m 52s
mir-libs (armhf)	25m 33s
mir-libs (arm64)	26m 23s
snap (canonical/mir-test-tools, amd64)	12m 28s
snap (canonical/mir-test-tools, armhf)	18m 22s
snap (canonical/mir-test-tools, arm64)	24m 29s
snap (canonical/confined-shell-wip, amd64)	8m 55s
snap (canonical/confined-shell-wip, armhf)	11m 54s
snap (canonical/confined-shell-wip, arm64)	13m 52s
snap (canonical/ubuntu-frame, amd64)	7m 28s
snap (canonical/ubuntu-frame, armhf)	12m 15s
snap (canonical/ubuntu-frame, arm64)	14m 25s
snap (Miriway/Miriway, amd64)	8m 52s
snap (Miriway/Miriway, armhf)	15m 18s
snap (Miriway/Miriway, arm64)	13m 56s
	3h 52m 3s

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