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

Issue #4889

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

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)