Problem:    Not enough testing for movement commands.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6313)
https://github.com/vim/vim/commit/bdd2c290d3cda69e0046c42f0c651f60bc510a16

Cherry-pick test_functions.vim changes from patch 8.2.0183.
Cherry-pick Test_normal18_z_fold() change from patch 8.2.0540.

Adds support for a bar at the top of each window, enabled through the
`'winbar'` option.

Co-authored-by: Björn Linse <bjorn.linse@gmail.com>

Problem:    getcharpos() may change a mark position.
Solution:   Copy the mark position. (closes vim/vim#10148)
https://github.com/vim/vim/commit/3caf1cce2b85a8f24195d057f0ad63082543e99e

Problem:    "char" functions return the wront column in Insert mode when the
            cursor is beyond the end of the line.
Solution:   Compute the column correctly. (Yegappan Lakshmanan, closes vim/vim#7669)
https://github.com/vim/vim/commit/9145846b6aa411e3ab5c0d145b37808654352877

Problem:    curpos() does not accept a string argument as before.
solution:   Make a string argument work again. (Yegappan Lakshmanan,
            closes vim/vim#7690
https://github.com/vim/vim/commit/9ebcf231bdccc1673cc92b20f5190fc577ad29d0

Problem:    Not easy to get mark en cursor posotion by character count.
Solution:   Add functions that use character index. (Yegappan Lakshmanan,
            closes vim/vim#7648)
https://github.com/vim/vim/commit/6f02b00bb0958f70bc15534e115b4c6dadff0e06

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
https://github.com/vim/vim/commit/f92e58cadb03156879e9bdbf6341bf662d9c87cc

Cherry-pick s:normalize_fname for tolower test from v8.1.0894 and v8.1.1417 (even though it is
unused for now).

Fix header for win_id2tabwin in eval.txt.

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
https://github.com/vim/vim/commit/aad222c9c9a1e4fe6ae5a1fe95bb084619be0e65

Partial port as this does not include eval.txt change for listener_add.

Cherry-pick eval.txt changes for:
- bufadd from v8.1.1626:
  https://github.com/vim/vim/commit/5ca1ac373ae62b37152cb6f85916b402eda520a5
- setloclist and setqflist headers from:
  https://github.com/vim/vim/commit/b0d45e7f5354375edd02afafde3bd37dac1515ff

Correct eval.txt typo for settabwinvar method call (matches latest Vim).

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
https://github.com/vim/vim/commit/196b4664432f932625cfb6371dc42c24efe6c203

server2client requires +clientserver, which hasn't been ported yet.
The eval.txt docs and test_clientserver.vim tests for server2client already exist, so include those
changes.

test_bufline.vim: Test for setbufline requires v8.1.1189 (which was reverted in #10848).