fixes #13078

Co-authored-by: Yuta Katayama <8683947+yutkat@users.noreply.github.com>

Problem:
Since right-click can now show a popup menu, we can provide messaging to
guide users who expect 'mouse' to be disabled by default. So 'mouse' can
now be enabled by default.

Solution:
Do it.
Closes #15521

Add 'mousescroll' option to control how many lines to scroll by when a
mouse wheel keycode is received. The mousescroll option controls both
horizontal and vertical scrolling. The option is a string in the format:

    set mousescroll=direction:count,direction:count

Where direction is either "ver" or "hor", and count is a non negative
integer. If a direction is omitted, a default value is used. The default
values remain unchanged, that is 3 for vertical scrolling, and 6 for
horizontal scrolling. As such, the mousescroll default is "ver:3,hor:6".

Add mousescroll documentation
 - Add option documentation in options.txt
 - Add brief summary in quickref.txt

Update :help scroll-mouse-wheel
 - Mention mousescroll option as a means of controlling scrolling.
 - Remove obsolete suggestion to map scroll wheel keys to <C-U> to
   scroll by a single line -- users should prefer the mousescroll option.
 - Add some information about the consequences of remapping scroll wheel
   keys (they lose their magic ability to affect inactive windows).

Update :help vim-differences
 - Add brief mousescroll summary under Options

Add mousescroll tests
 - Test option validation
 - Test default mousescroll value and behavior
 - Test fallback to default values
 - Test mouse vertical and horizontal scrolling in normal mode
 - Test mouse vertical and horizontal scrolling in insert mode

Neovim already removed `evim` (or any similar flags). The 'insertmode'
option is a weird remnant, so get rid of it.

The 'insertmode' option is replaced with a script that closely emulates
the option. This script is documented at :help 'insertmode'

Having two methods of setting the same option is redundant and can be
potentially confusing. Recommend adding 's' to 'shortmess' instead.

An option that should always be kept on except for when working with old
Vi scripts is of little use to us.

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:    C++ scope labels are hard-coded.
Solution:   Add 'cinscopedecls' to define the labels. (Tom Praschan,
            closes vim/vim#10109)
https://github.com/vim/vim/commit/3506cf34c17c5eae6c2d1317db1fcd5a8493c288