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

[Juri Linkov]

>> This patch checks for existing face properties to not overwrite them:
>
> I'm not really convinced that there should be any face properties on the
> minibuffer messages at all, though.  :-)

Without highlighting of minibuffer messages there is a danger that the
message will remain unnoticed, especially in the multiline minibuffer.

So minibuffer messages need to be highlighted for the same reason
why the minibuffer prompt is already highlighted:

1. to designate non-editable read-only parts of the minibuffer;

2. to help the users to turn attention to the active part
   of the minibuffer.

Thus all informational text in the minibuffer should be highlighted
consistently in one color to help not to miss important message.

Isn't it so already that elsewhere in Emacs font-lock is used
to highlight more important parts of the buffer?

PS: I already added configurability in the previous patch,
so now we are discussing the default value.