- Work with a bool[] array parallel to the UIWidget enum.
- Rename some functions.
- Documentation.

Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:

1. msgpackparse() will show internal error: hash_add() in case of duplicate
   keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
   expected. Test was still functioning somehow though. Currently fixed.

Helped-by: Matthew Malcomson <hardenedapple@gmail.com>

Closes #6451

Also fixes buffer reusage in setmatches() and complete().

Problem:    Cannot get all information about a quickfix list.
Solution:   Add an optional argument to get/set loc/qf list(). (Yegappan
            Lakshmanan)

https://github.com/vim/vim/commit/d823fa910cca43fec3c31c030ee908a14c272640

Problem:    Arguments of setqflist() are not checked properly.
Solution:   Add better checks, add a test. (Nikolai Pavlov, Hirohito Higashi,
            closes vim/vim#661)

https://github.com/vim/vim/commit/d106e5ba7f10f0d2a14eaefe5d78405044416cb9

https://github.com/mpeterv/luacheck/pull/81#issuecomment-261099606

> If you really want to use bleeding-edge version you should get the
> rockspec from master branch, not a fixed commit ...
> The correct way to install from a specific commit is cloning that
> commit and running "luarocks make" from project directory. The reason
> is that running "install" or "build" on an scm rockspec fetches
> sources from master but uses build description from the rockspec
> itself, which may be outdated.

Problem:    When using Insert mode completion but not actually inserting
            anything an undo item is still created. (Tommy Allen)
Solution:   Do not call stop_arrow() when not inserting anything.