Size of xterm.c doubled since Emacs 28

[Stefan Kangas]

Hi,

I have noticed that xterm.c has seen a lot of activity recently, and
increased in size from ~14,000 lines to ~28,000 lines of code between
emacs-28 and master.  I wanted to ask if parts of that file should
perhaps better be put in one or more separate files, for the usual
reasons.

For reference, here is a list of our 10 longest files in src:

   1.   xdisp.c       37099
   2.   xterm.c       28422
   3.   keyboard.c    13197
   4.   coding.c      12241
   5.   w32fns.c      11273
   6.   image.c       12194
   7.   xfns.c        10048
   8.   w32.c         11132
   9.   window.c       8707
   10.  process.c      8795

Thanks.