Provides an ICommandButtonRegistry to descendants. When no provider is present the shared
DefaultCommandButtonRegistry singleton is used (see useCommandButtonRegistry), so
this is purely additive - a host may wrap a subtree to override contributions without affecting
anyone still reaching for the singleton directly.
Provides an ICommandButtonRegistry to descendants. When no provider is present the shared DefaultCommandButtonRegistry singleton is used (see useCommandButtonRegistry), so this is purely additive - a host may wrap a subtree to override contributions without affecting anyone still reaching for the singleton directly.