bug#15899: 24.3.50; regression: `region' overlay is lower priority than default

[Drew Adams]

> It has a command `easy-mark' which selects some unit of text around
> point. And it uses a dedicated overlay to mark the place where point
> was before the command was called, in color.
> 
> So that overlay needs to have higher priority than region. No need
> to make region highlighting invisible.

Similarly, the isearch overlays should have higher priority than any
region overlay.

(And yes, I agree with you that users should of course be able to
change priorities as they see fit.)

If region highlighting is to use an overlay, then its default priority
should be higher than most predefined (i.e., emacs -Q) overlays, but
lower than the isearch priorities.