emacs: notmuch-show: refresh all windows displaying buffer

emacs: notmuch-show: refresh all windows displaying buffer

This updates all windows displaying a notmuch-show buffer when the
buffer refresh function is called.

Each window displaying a notmuch-show buffer has its own currently
displayed message based on the (point) location. We store the state
of all displayed windows when refreshing a notmuch-show buffer and
re-apply the current shown message (point) for all windows.

Implementation note: Each window has it's own (point) value, besides
the buffer's (point) value. Sometimes these values are identical like
in the case where a single window displays a buffer. When multiple
windows display a buffer, (point) returns each window's specific value.
What we are storing in this changeset is the window values not the
buffer point values. The buffer's point is returned only if no window
is displaying the buffer, a case we do not care about here.

Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>