bug#28771: 26.0.60; A couple space display property feature requests

[Alex <agrambot@gmail.com>]

I've branched this out from the following thread instead of posting it
there:
https://lists.gnu.org/archive/html/help-gnu-emacs/2017-10/msg00037.html

Here's a few features that would be nice for the pixel-specified space
properties.

1. Specifying the pixel width to the left/center/right of the text area.
   This is helpful (I believe it's necessary if I want it to be robust)
   when one wants to pad either side of a string with the same number of
   pixels and the left side uses (:align-to 0).

   I could use (:width (+ left-fringe left-margin)) for the right
   padding, but this doesn't take into account line-number display, as
   well as the scroll-bar width (if it's on the left).

2. Specifying the pixel position relative to the left or right edge of
   the window. The right edge of the window currently may be the right
   edge of the right fringe or the scroll bar, depending on what side it
   is on. Also, just in the case that in the future the
   fringe/scroll-bar may not be the outermost elements, there should be
   a way to explicitly specify one of the edges.

3. Following from "1." and the help-gnu-emacs thread, it would be nice
   if there was a display property that allowed you to pad a string on
   both sides with a pixel-specified space instead of surrounding it
   with 2 pixel-specified spaces.