bug#34506: 27.0.50: push-button bug with basic text-property button

[Robert Weiner]

> On Feb 19, 2019, at 10:26 AM, Basil L. Contovounesios <contovob@tcd.ie> wrote:
> 
> I don't see why anyone
>   would mix widgets and buttons in the same buffer and run the risk of
>   such gotchas.  I believe this contributed in part to Robert's
>   confusion.

If buttons and widgets are wholly incompatible and separate then button-at 
should return nil when on a widget even if both buttons and widgets use the 
same underlying mechanism.  This is what abstractions are for.  The 
implementation should be hidden.

The abstraction I really want is if there is a clickable item under point, 
activate it in the most appropriate way, e.g. if selected with a mouse event, 
use it’s mouse action, etc.  Maybe an activate-at function that handles 
different types of objects with a priority order if there are multiple matches. 

Bob