X11: Added reliable detection of autorepeat for Xkb

[jpcima]

This is a proposal for a reliable detection of repeated keys for Linux.
It is not based on peeking a future event and its timestamp.

Method

Use of libX11 API XkbSetDetectableAutoRepeat.

Since the server-side may not necessarily support the request, it's good to keep the basic method as a fallback.

The purpose is to disable the reception of release events for repeated keys.
Then, if there is a press on a key already marked as down in the key state, it is identifiable as a repetition.

So unlike the basic method which filters out the release events, this filters the key press.

Description of problem

It solves a problem where repetitions are not detected in all cases.
A notable symptom is the MidiKeyboardComponent playing its notes in rapid fire, while keeping pressed one or more of the keys.

Additional information

By the currently used method, the repetitions are detected by timestamp on the press event which follows a release.

A problem is that there is a race for the reception of the press event.
The call XPending of isKeyReleasePartOfAutoRepeat can be observed returning false when the second event is not available in time.

This is a custom trace which shows the event sequence and the information around autorepeat.
trace.diff.gz

KeyPress code=42 time=195687039
KeyRelease code=42 time=195687051
- AutoRepeat: pending event KeyPress
- isKeyReleasePartOfAutoRepeat: true
KeyPress code=42 time=195687051
KeyRelease code=42 time=195687064
- AutoRepeat: pending event <none>
- isKeyReleasePartOfAutoRepeat: false
KeyPress code=42 time=195687064
KeyRelease code=42 time=195687077
- AutoRepeat: pending event KeyPress
- isKeyReleasePartOfAutoRepeat: true
KeyPress code=42 time=195687077

Environment of test

I am running this software: current Arch Linux, X.Org 1.20.0, MATE Desktop 1.20.3.
Linux quad 4.16.18-rt9-1-rt #1 SMP PREEMPT RT Fri Jul 13 21:57:24 CEST 2018 x86_64 GNU/Linux