Problem:    The screen.c file is much too big.
Solution:   Split it in three parts. (Yegappan Lakshmanan, closes vim/vim#4943)
https://github.com/vim/vim/commit/7528d1f6b5422750eb778dfb550cfd0b0e540964

This is an approximation vim-patch 8.1.2057. Applying the patch directly
isn't feasible since our version of screen.c has diverged too much,
however we still introduce drawscreen.c and drawline.c:
- screen.c is now a much smaller file used for low level screen functions
- drawline.c contains everything needed for win_line()
- drawscreen.c contains everything needed for update_screen()

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Mostly avoids including eval.h, ex_cmds2.h and ex_docmd.h in other
headers.

Problem:    Default cmdwin mappings are re-mappable.
Solution:   Make the default mappings not re-mappable. (closes vim/vim#10580)  Use
            symbols for the first do_map() argument.
https://github.com/vim/vim/commit/44068e97dbd8fc8ebd93113e436a1e37a6bff52c

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459

This marks the following Vim patches as ported:

vim-patch:8.1.1785: map functionality mixed with character input

Problem:    Map functionality mixed with character input.
Solution:   Move the map functionality to a separate file. (Yegappan
            Lakshmanan, closes vim/vim#4740)  Graduate the +localmap feature.
https://github.com/vim/vim/commit/b66bab381c8ba71fd6e92327d1d34c6f8a65f2a7

vim-patch:8.2.3643: header for source file is outdated

Problem:    Header for source file is outdated.
Solution:   Make the header more accurate. (closes vim/vim#9186)
https://github.com/vim/vim/commit/a3f83feb63eae5464a620ae793c002eb45f7a838

Also cherry-pick a change for <unique> mappings from patch 8.2.0807.
Rename map_clear_mode() to do_mapclear().

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459