Using the attribute name semantics proposed in T97452, this patch moves
the hide status of mesh elements from the `ME_FLAG` of vertices, edges,
and faces to generic boolean attribute layers. Storing this data as
generic attributes can siginificantly simplify and inprove code,
as described in T95965.
The attributes are called `.hide_vert`, `.hide_edge`, and `.hide_face`.
The `.` prefix means they are "UI attributes", so still contain
original data edited by users, but they aren't meant to be accessed
procedurally by the user in arbitrary situations. They will also be
hidden in the spreadsheet and the attribute list by default (I left
that change out for now to help the review process).
Branch: `refactor-mesh-hide-generic`