Snap

Add APIs to override decoration managers and customize decorations (Take 2) #4961

Job	Run time
arches	1s
mir-libs (amd64)	19m 11s
mir-libs (armhf)	25m 9s
mir-libs (arm64)	24m 39s
snap (canonical/mir-test-tools, amd64)	12m 11s
snap (canonical/mir-test-tools, armhf)	18m 16s
snap (canonical/mir-test-tools, arm64)	21m 45s
snap (canonical/confined-shell-wip, amd64)	6m 30s
snap (canonical/confined-shell-wip, armhf)	14m 35s
snap (canonical/confined-shell-wip, arm64)	13m 9s
snap (canonical/ubuntu-frame, amd64)	7m 15s
snap (canonical/ubuntu-frame, armhf)	12m 56s
snap (canonical/ubuntu-frame, arm64)	14m 28s
snap (Miriway/Miriway, amd64)	7m 46s
snap (Miriway/Miriway, armhf)	13m 41s
snap (Miriway/Miriway, arm64)	15m 10s
	3h 46m 42s

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