Issue #4307

As per CommonMark (conflicting with markdown.pl, but never
mind markdown.pl)

Closes #4395

when accessing `document.activeElement` from inside an iframe.

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

Rather than on read

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().

As described in http://caniuse.com/#feat=user-select-none and https://developer.mozilla.org/en-US/docs/Web/CSS/user-select

Issue #4374

 - Added for as a defining keyword
 - Added new types and operators
 - Fixed numbers so basic integers are represented
 - Identifiers can now be surrounded with backticks
 - Properties and #/@ instructions are now distinct, with the latter represented as a builtin type
 - Properties are now matched before punctuation. Code can now fold.
 - Remove the regexp checking as that syntax does not currently exist in Swift
 - Added tests

So that their vertical offsets actually match the position of the
corresponding lines.

Issue #4364

And use it in the merge addon to get the proper offsets for merge buttons

Issue #4364

Issue #4353

Issue #4353

Closes #4353

and adds the xù specific keyword 'xu'

Closes #4345

Demo had the correct `text/x-vue`
defineMIME was using `script/x-vue`

There is, however, no `script/*` top-level media type:
http://www.iana.org/assignments/media-types/media-types.xhtml

Left, old `script/x-vue` for backwards compatibility.

Closes #4340