Support full entity-path-expressions for overrides

[jleibs]

• Introduce new python API for setting overides #6650 introduced support for overriding components on entities

However, this implementation only supports explicit fully qualified entity paths.

We would like the keys of overrides to support arbitrary path expressions including $origin and /** glob values.

Accomplishing this requires:

• Refactoring the view blueprint storage to be a flat list of Overrides where each override contains an EntityPathExpression component indicating where the override applies.
• Fixing the override resolver to do find the most-specific override that matches the entity.
• Making updates to the UI for the override stack to reflect all the possible matches in priority order.