bug#29279: Sharing the margins

[Eli Zaretskii <eliz@gnu.org>]

I tend to like better the proposal originally made by Joost Kremers in
http://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01540.html.
Here it is with slight variations of my own:

We add 3 new functions:

 (defun window-margin-add (window side symbol cols &optional ordinal)
   "Request allocation of COLS columns of WINDOW's marginal area.
 WINDOW must be a live window and defaults to the selected one.
 SIDE is the window side, either `left' or `right', in whose margin
   the allocation is requested.
 SYMBOL identifies the request, in case the allocation needs to be
   changed or deleted.
 ORDINAL is the optional ordinal number of the requested area, counted
   from left to right.  Negative ordinal numbers count from right to
   left.  Zero means the value of COLS is the maximum width of the
   marginal area, and no separate allocation is requested.

 Value is the actual positive ordinal number of the allocated area."

 (defun window-margin-remove (window side symbol)
   "Remove a previously allocated part of WINDOW's marginal area.
 WINDOW must be a live window and defaults to the selected one.
 SIDE is the window side, either `left' or `right', in whose margin
   the removal is requested.
 SYMBOL identifies the request to be deleted."

 (defun window-margin-modify (window side symbol cols)
   "Modify a previously allocated part of WINDOW's marginal area.
 WINDOW must be a live window and defaults to the selected one.
 SIDE is the window side, either `left' or `right', in whose margin
   the modification is requested.
 SYMBOL identifies the request to be modified."

The details of the allocated portions of the marginal area are
maintained in window parameters `left-margin-params' and
`right-margin-params', which are modified by those 2 functions.  The
display engine uses these parameters to calculate the number of the
portions and the width of each one of them, and pre-allocates them in
the window's glyph matrices whenever a redisplay follows a change in
margin allocation.  The display spec for display in the margins will
include the symbol of the allocated portion pertaining to the display
spec.  Like this:

  '(display ((margin left-margin 'toto) STRING))

If the stuff (text, image, etc.) to be displayed in a specified
portion of the marginal area is too wide and doesn't fit the width of
the portion, it will be truncated; if it is narrower, it will be
padded.

This keeps the display engine design intact, and I think will allow
relatively straightforward implementation (although some non-trivial
changes in management of the glyph matrices will be necessary).  It
can also be implemented in a way that keeps backward compatibility
with the current APIs.

WDYT?