bug#74423: Low level key events

[Cecilio Pardo]

Here is a new version of the patch. Sorry it took so long. It
addresses most of the notes I got:

Notes by Po Lu:
  - I think now the associated modifier for a key is computed
    properly, with or without XKB for plain X, and for gtk.
  - 'ralt' not generated for ISO_Level3_Shift. This will have to stay
    like this. What we know about the keys if what the system lets us.
  - Don't call x_any_window_to_frame if already called for the event.

Notes by Eli:
  - Change testing for Venable_low_level_key_events. This variable now
    is more complex.

Notes by Stefan:
  - Created and used event-* functions to access event.
  - Only one event with both pieces of information.
  - Selection of keys. Now the variable enable-low-level-key-events
    can be used to select exactly which keys we want to process.
  - About creating lisp object in handle_one_xevent, it is done in
    other events too.

The lisp interface is as follows:
  - A list of variables with keysym values is initialized (xk-*), so the
    user can select keys, not just modifiers.
  - Users can bind globally commands or functions with llk-bind to taps.
  - To use a tapped key as a new modifier, this works:
    (llk-bind 'tap 'xk-shift-r (lambda ()
               (message "H-...")
               (setq unread-command-events
                  (append (event-apply-hyper-modifier nil) nil))))
  - Added the function describe-low-level-key to see what
    keysym/modifier it triggers.


This is implemented for X, pgtkw and MS-Windows.
The pgtj implementation detects that a key is a modifier to filter it, 
but can't decide which one it its.

0001-Send-event-for-key-presses-and-key-releases.patch
Description: Text document