feat (a11y): support a11y protocol v3 and show color filter toggle even on v2

[JasonQiu1]

Support a11y protocol v3. Show color filter toggle on v2 only after the first time it has been set somewhere else before, since it'll work as expected then.

Depends on pop-os/cosmic-settings#2054 for updated cosmic-settings-a11y-manager-subscription package.

Should help issue #928.

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