gui: automatic Observable names

[Bogdanp]

It would be nice if the GUI library named observables based on the name of the definition each Observable is attached to (i.e. an analogue of define/obs from gui-easy-lib). One way Observable names are used is in the (undocumented 😅) debugger. For example, running the following command in the rhombus repo root

$ racket -l racket/gui/easy/debugger -t gui_demo.rhm

(requires an up-to-date gui-easy-lib)

runs the GUI demo with a separate debugger window where you can see observable updates as they occur. Now and then, this can come in handy when debugging problems. The issue right now is under typical uses of rhombus-gui, every observable is going to be named anon. Another issue is reflected observables don't also reflect the original observable's name. The latter is easy to fix, but I'm not yet familiar enough with Rhombus to know if automatic naming is achievable using standard syntax.

Not yet supported, but one thing I'd like to do eventually to make the debugger more useful is to also store source location information on the observable itself (probably by adding a #:source keyword argument to obs). That would make it possible for us to support something like a context menu on the debugger table to allow the user to jump to that observable in their editor. So that's also something to consider.

[mflatt]

Some initial steps:

I've added Obs.def and Obs.let. They're like define-obs — without the special case when the right-hand side is an observable, so far, because I don't quite seem how that twist helps.

Reflecting observables now have names that match their properties. I didn't make them use the name of a given observable, because they seem to me distinct from those observables. The name might be improved by including the view class that was instantiated, but I'm not sure that would be better, either, so I opted for a shorter immediate name.