Before the autocommands TextYankPost was not firing with userregs, now
it does. This also fixes the multi-byte encoding of the userreg in the
v:event dict.

feat: multibuffer preview support for inccommand

Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
https://github.com/vim/vim/commit/b340baed9f7fc1c19a0742e2214d54982190c15e

Omit test_override().
Reorder test_writefile.vim to match Vim.

Problem:    Code for modeless selection not sufficiently tested.
Solution:   Add tests.  Move mouse code functionality to a common script file.
            (Yegappan Lakshmanan, closes vim/vim#5821)
https://github.com/vim/vim/commit/515545e11f523d14343b1e588dc0b9bd3d362bc2

Skip termcode tests for now.

Problem:    Cannot read error message when abort() is called.
Solution:   Output a newline before calling abort().
https://github.com/vim/vim/commit/213e70e284b0975dd34525e94e59e26811097c72

Add emsg_not_now() check to make code equivalent.

Problem:    When an internal error makes Vim exit the error is not seen.
Solution:   Add the error to the test output.
https://github.com/vim/vim/commit/1c67f3a9779b99bed7aacb3108abbb649445d3ed

Add emsg_not_now() check to make code equivalent.

Problem:    Some code in options.c fits better elsewhere.
Solution:   Move functions from options.c to other files. (Yegappan
            Lakshmanan, closes vim/vim#4889)
https://github.com/vim/vim/commit/e677df8d93772a705f40a94f3c871aee78fe4d99

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>