Add APIs to override decoration managers and customize decorations (Take 2) #4966

Job	Run time
arches	1s
mir-libs (amd64)	18m 41s
mir-libs (armhf)	25m 22s
mir-libs (arm64)	24m 26s
snap (canonical/mir-test-tools, amd64)	13m 8s
snap (canonical/mir-test-tools, armhf)	19m 32s
snap (canonical/mir-test-tools, arm64)	24m 15s
snap (canonical/confined-shell-wip, amd64)	7m 47s
snap (canonical/confined-shell-wip, armhf)	14m 45s
snap (canonical/confined-shell-wip, arm64)	12m 56s
snap (canonical/ubuntu-frame, amd64)	6m 53s
snap (canonical/ubuntu-frame, armhf)	13m 56s
snap (canonical/ubuntu-frame, arm64)	15m 44s
snap (Miriway/Miriway, amd64)	7m 19s
snap (Miriway/Miriway, armhf)	19m 56s
snap (Miriway/Miriway, arm64)	17m 52s
	4h 2m 33s

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