T200915: Allow SlotRoleHandlers to control page layout

Phabricator Link	T200915
Status	open
Priority	needs triage (violet)
Author	[1]
Owner
Projects	Multi-Content-Revisions (New Features) CPT Initiatives (MCR)
Subtasks	T194046: Introduce SlotRoleHandler and SlotRoleRegistry for declaring slot roles., T205444: Provide a way of having a meaningful slot header, T211726: Collect requirements for slot based layout, T194048: Introduce RevisionRenderer (baseline)
Parent Tasks	T209878: Allow control of page layout of multiple slots during rendering
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