Snap

Add APIs to override decoration managers and customize decorations (Take 2) #4978

Job	Run time
arches	1s
mir-libs (amd64)	17m 47s
mir-libs (armhf)	26m 4s
mir-libs (arm64)	26m 14s
snap (canonical/mir-test-tools, amd64)	12m 30s
snap (canonical/mir-test-tools, armhf)	19m 21s
snap (canonical/mir-test-tools, arm64)	21m 51s
snap (canonical/confined-shell-wip, amd64)	7m 10s
snap (canonical/confined-shell-wip, armhf)	16m 23s
snap (canonical/confined-shell-wip, arm64)	12m 45s
snap (canonical/ubuntu-frame, amd64)	7m 40s
snap (canonical/ubuntu-frame, armhf)	14m 49s
snap (canonical/ubuntu-frame, arm64)	17m 0s
snap (Miriway/Miriway, amd64)	8m 40s
snap (Miriway/Miriway, armhf)	17m 53s
snap (Miriway/Miriway, arm64)	17m 12s
	4h 3m 20s

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