bug#28194: text-scale-adjust should show current accumulated scale factor

[積丹尼 Dan Jacobson]

C-x C-= runs the command text-scale-adjust.

It should give some feedback, in the minibuffer,
about the current accumulated scale factor etc.
E.g., the user repeats the command using '=', but nothing on the screen
changes until each fifth '='. In the meantime a running tally of the
current accumulated scale factor could be shown in the minibuffer so the
user knows the command is indeed doing something.

Also when the fonts get bigger (e.g., every fifth '=') have a message
saying what new font has been used.