Unfeature: disallow "*" in a nonterminal position in the observe mini-language

[mdickinson]

PR #1496 introduced "*" to mean "any trait" in the observe mini-language. But a "*" in anything other than a terminal position (for example, as in my_obj.observe(handler, "*:updated") is hard to use, thanks to a combination of two things:

• it picks up all traits, including weird things like trait_added and trait_modified, and so typically includes at least some traits for which the following expression won't apply well - trait_added is an Event trait, and its value doesn't have an updated trait
• the observe framework is (intentionally) nitpicky about invalid trait names: on_trait_change won't complain about trying to set up a listener for the updated trait on an object that doesn't have an updated trait (and possibly isn't even a HasTraits instance); observe will.

There may be a better meaning for "*" in a non-terminal position - for example, we might consider that after a "*", the following expression is treated permissively (using something like the existing optional=True).

For now, we make a "*" in a non-terminal position illegal, so that we have the freedom to re-add it later with better behaviour. (Note that the "*" feature has not yet appeared in any released version of Traits, so there are no backwards compatibility issues here.)

Closes #1505
Related: #1503

[corranwebster]

Looks good.

Are there changes to documentation that are needed? Or are you planning those in a separate PR?

Also could you add an issue about exploring * in non-terminal positions - particularly the suggestion for having the exist flag false in that situation - with a link to this PR so we have a place to capture any discussion and context.

[mdickinson]

Are there changes to documentation that are needed?

Good point; done. All of the examples added to the docs in #1496 are still valid, so there's no change needed there.

Also could you add an issue about exploring * in non-terminal positions

Done: #1526