Keypress should not happen during composing (as spec) but it does on Windows for Korean IMEs

Closes #4878.

Issue #4889

Modern browsers handle updates to a textarea's value fast enough
to not do this anymore.

Issue #2703

Issue #4800

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 #4726

Issue #4625

(when there is a gutter)

Issue #4637

Which often caused strangeness around empty lines

Issue #4307

So that we don't leave a stray selection that'll be reset
to some nonsense position by the DOM changes.

Closes #4625

This is based on work by Radek Piórkowski <radek25c@gmail.com>.

Closes #4589

Closes #4569

when forced after compositionend event.

Issue #4441

Issue #4307

So that the code doesn't get confused when backspacing or
deleting across a line.

This is still flaky. Ideally we'd capture backspace as a key event,
but Android Chrome makes that impossible.

Issue #4307

And do so only after a delay, so that subsequent input
events get a chance to fire.

Because Android, especially Google Keyboard, just doesn't care

So that the selection isn't reset to the start of the element by
div.focus().

When selecting at the beginning of a line ranges need to be removed
for new range to be added properly. (in Safari)