Implementation note: This will probably use frame-find-innermost-applicable-presentation-
at-position to locate the innermost presentation, and unhighlight-highlighted-presentation
and set-highlighted-presentation to unhighlight and highlight presentations.

=> set-highlighted-presentation stream presentation &optional prefer-pointer-window [Function]
Highlights the presentation presentation on stream. This must call highlight-presentation
methods if that is appropriate.

=> unhighlight-highlighted-presentation stream &optional prefer-pointer-window [Function]
Unhighlights any highlighted presentations on stream.

23.7.4 Translator Applicability

The top-level "input wait", which is what you are in when inside of a with-input-context,
is responsible for determining what translators are applicable to which presentations in a given
input context. This loop both provides feedback in the form of highlighting sensitive presentation,
and is responsible for calling the applicable translator when the user presses a pointer button.

Implementation note: with-input-context uses frame-find-innermost-applicable-presentation-
at-position (via highlight-applicable-presentation) as its "input wait" handler, and
frame-input-context-button-press-handler as its button press "event handler".

Given a presentation, an input context established by with-input-context, and an event cor-
responding to a user gesture, translator matching proceeds as follows.

The set of candidate translators is initially those translators accessible in the command table in
use by the current application. A translator is said to "match" if all of the following are true (in
this order):

1. The presentation's type is presentation-subtypep of the translator's from-type, ignoring
   type parameters.
   
   
2. The translator's to-type is presentation-subtypep of the input context type, ignoring
   type parameters.
   
   
3. The translator's gesture is either t, or matches the event corresponding to the user's
   gesture.
   
   
4. If there are parameters in the from-type, the presentation's object must be presentation-
   typep of the from-type.