Use ContextValues consistently to bind editor values

In the `VideoEditor` we are experimenting with using `ContextValues` to bind editor traits to values in the UI Context. If this works well, we would like to roll that out to other places where it would make sense, such as `high` and `low` traits of the `RangeEditor`.

Doing this will require:

- identifying the places which can make use of this construct (there may be many more appropriate traits than currently exposed, due to previous complexity of setting this up)
- for each trait:
- make the trait on the factory a `CVType(...)` trait or similar
- add appropriate `sync_value` metadata to the trait
- remove any explicit calls to `Editor.sync_value` and let the automatic metadata connection handle things
- (for backwards compatibility) add `sync_name` metadata where appropriate to allow use of old `*_name` traits
- document the `ContextValue` mechanism in a more prominent way
- (eventually) deprecate older `*_name`-style traits
- (even later) remove `*_name`-style traits which are no longer needed.

The end goal should be that when a user asks how they make an attribute of an editor dynamic, the answer is simply "use a `ContextValue` binding" and it generally works.

Start by examining the existing ContextValues usage in VideoEditor and compare it with traits such as high and low in RangeEditor. Then identify other editor traits and their current synchronization paths. Done means appropriate traits use the binding mechanism consistently, backwards compatibility is addressed, and the ContextValue mechanism is documented more prominently.

Written by the indexing model from the issue text.