(see grzegorzmazur/yacas#231)

Closes #4785

Closes #4786.

Enums are available in Dotty and soon will be available in Scala.

Addresses #4771

Closes #4774

Closes #4773

So that Doc can do its auto-new trick before calling the
superclass constructor.

Closes #4768

Rather than abusing variable-3. Coloring is the same for now.

Since the vim bindins are depending on that

Closes #4760

FEATURE: Mouse button clicks can now be bound in keymaps by using
names like `"LeftClick"` or `"Ctrl-Alt-MiddleTripleClick"`. When bound
to a function, that function will be passed the position of the click
as second argument.

FEATURE: The behavior of mouse selection and dragging can now be
customized with the
[`configureMouse`](http://codemirror.net/doc/manual.html#option_configureMouse)
option.

Issue #4727

Issue #4759

See https://discuss.codemirror.net/t/nested-block-comments-not-displayed-correctly/1253/3

See https://discuss.codemirror.net/t/how-to-check-if-inside-comment/1252

This closes #4691 and is based on an idea by Joel <joel.einbinder@gmail.com>.

This fixes keyword highlighting for CPython and Verilog.

Most cases look like simple typos, other cases suggest misunderstanding of regular expression syntax.

Issue #4754