bug#19931: 24.4; Support for OS X press and hold

[Juri Linkov]

>> I don't know how "Press and hold" works on OS X, but it seems it's like
>> on some smartphone screen virtual keyboards holding down a letter pops up
>> a list of suggestions with accented letters.
>
> Yes exactly similar to that.

I noticed there is already the corresponding item in etc/TODO:

  **** PressAndHold for writing accented character
  On macOS, many application support the press and hold pattern to
  invoke a menu of accented characters.  (See example at
  https://support.apple.com/en-us/HT201586 .)

  Currently, this doesn't work in Emacs.

  Note that "ns-win.el" explicitly disables this.

  Note: This feature might not be allowed to be implemented until also
  implemented in Emacs for a free system.