Replace dynamic uses of on_trait_change with observe part 2

[aaronayres35]

part of #416

This PR finishes the job started in #656 by replacing the remaining dynamic uses of on_trait_change with observe.

[aaronayres35]

BEWARE: extended_name

[jwiggins]

Meaning what, exactly? We can't be sure that name won't break when passed to observe?

[aaronayres35]

Yeah, a quick search of the code base for extended_name only gives this result here and the corresponding same thing in the wx file. Any downstream users that set this trait may run into problems if the extended name they are passing does not conform to the new mini language, and that is effectively out of our hands ...

A similar scenario was encountered in pyface and we mentioned it in the release announcement. I imagine the vast majority of users would not be affected (I dont even know how many users of this code we have in total). Nonetheless, it leaves a door open for potential breaks.

Another thing to worry about here though that I'm not sure what we would want to do for: The traitsUI code for Editor contains the code that would theoretically remove this listener (in either the dispose method or the _update_editor method itself. I also just realize I made a dumb mistake! The method being observed is _update_editor not update_editor which is defined in traitsUI. We are still in the process of updating traitsUI to observe, so that method will need its signature changed... I believe we will need that around first, before we can make this change as this code won't work currently. It must be uncovered in the test suite hence we don't see a failure here

[jwiggins]

LGTM. The test for AbstractWindow looks fine.