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:

BTW, there are more related items in etc/TODO.  I don't know
how actual they are.  Maybe some of them are already obsolete?

  ** Add a way to define input methods by computing them
  When an input method is first used, redefine C-x 8 to use a
  user-selected input method, with the default being the union of
  latin-1-prefix and latin-1-postfix.

  ** A more convenient use of input methods in search
  Arrange a way for an input method to return the first character
  immediately, then replace it later.  So that C-s a with input method
  latin-1-postfix would immediately search for an a.

  ** Let LEIM handle the Mode_switch key like XIM does
  I.e. a toggle like C-\ but which can also be used as a modifier.