bug#21112: 25; Patch: show minibuffer messages with a face

[Drew Adams <drew.adams@oracle.com>]

> For consistency with isearch messages that are highlighted with the
> minibuffer-prompt face, minibuffer-message needs to do the same.

Neither Isearch prompts nor `minibuffer-message' output
should be highlighted with face `minibuffer-prompt'.

Isearch doesn't use the minibuffer.
And `minibuffer-message' is not a prompt.

> But since a message is displayed at different part of the minibuffer,
> it should be highlighted using a new separate customizable variable
> similar to minibuffer-prompt-properties:

1. If you go that route, which lets libraries and
users easily change the properties (good), please
separate Isearch and `minibuffer-message'.  Use two
different variables.  They are not the same species.

The default value could be, but it need not be,
the same for both.

On the other hand, it could be helpful to separate
out property `face', defining an actual face (for
each of Isearch and `minibuffer-message') that
users can customize for this.  That also has the
benefit that one can inherit from one to the other.

The other properties would be applied first, so that
if `face' is among them for some reason it will be
overridden by the face provided specifically for this.

2. The doc string of `minibuffer-message-properties'
should not say that the properties are added to
"minibuffer messages" - that's too broad.  It should
say that they are added to the text shown by function
`minibuffer-message' - or else describe that text,
and only that text, in another way.

The doc string of the similar variable for Isearch
should say that the properties are added to "the
search prompt" or some such.  Not sure whether the
right description is "prompt", but whatever is the
best way to describe it should be used in the doc
string.