Unified mouse-highlight code needs testing

[Eli Zaretskii <eliz@gnu.org>]

I've committed a few minutes ago to the trunk revision 102261, which
unifies the mouse-highlight code for all sessions, including GUI
sessions and text terminals that support the mouse (GPM and MSDOS).
There's now only one set of note_FOO_highlight functions (in xdisp.c)
and their subroutines that determine which parts of the displayed text
should be highlighted.  The only display-specific part is a single
function which actually draws a given glyph row in mouse-face.

I tested this on MS-Windows (as a representative of GUI sessions) and
on MSDOS.  I don't have access to a system with GPM which actually has
a mouse, so I could only make sure it builds with GPM and runs
correctly without a mouse.  I also made sure the X build compiles and
links.

Please test the new code on a TTY with GPM, on X, and on NS.  On a TTY
with GPM, the immediate bonus from this unification is that now mode
line, header line, and overlays that specify mouse-face should all be
highlighted when the mouse hovers above them.  The easiest way of
testing this is in Info: you have there the mode line, the header
line, and the bread crumbs which all specify mouse-face; these should
all work now.

It is also important to test this in a session that has both X and TTY
frames, especially if the TTY frame uses GPM.

Thanks in advance.