Make the `focus` method shift focus to a floating frame. Also, shift
focus away from the floating frame when `focus` is used in an window
within the current main frame.
Previous versions of OS X seem to have automatically disabled
update for a window when flushing is disabled, but now we have to
do it ourselves, otherwise text editors can flicker a lot. (The
system change is probably related to the removal of the
`isFlushWindowDisabled` method of `NSWindow` (which was
deprecated in 10.10).
In particular, restore key handling that makes Ctl-Space produce an
event with a Space character. Also, restore `special-option-key`
support, while correcting the documentation to say what
`special-option-key` and `special-control-key` actually do.
Closes PR 14934
