Many of the plugins depend on the libX11 and libXi linker flags, but
they are missing.

With GNU libtool the -no-undefined flag is silently ignored so the build
succeeds, but with slibtool is correctly sets -Wl,--no-undefined in the
linker flags.

Gentoo issue: https://bugs.gentoo.org/922326

Fix fairly large memory leak when beeping on keys while caps lock is
enabled.  The libatspi2 docs and API were quite misleading, so I
overlooked the fact the event parameter should be freed in the
callback.

This changes the constness of the callback argument, which is new in
libatspi2 2.40 -- yet the actual behavior didn't change, only the
qualifier was removed, see [1].
This might however bring up a compiler warning when building against
libatspi2 < 2.40;  but on the other hand it fixed build with
clang >= 16, see #399.  As it is unlikely to build with clang >= 16
and libatspi2 < 2.40, I think it's a good compromise.

[1] https://gitlab.gnome.org/GNOME/at-spi2-core/-/commit/7dfb0b7fc2d1710ef7fad54f910fa4c6a5e3af17

Delay registration of the a11y keyboard plugin to run after the
non-a11y keyboard one to avoid numlock state change conflicts.

This allows a customizable and possibly different beep sequence when a
togglekey is enabled or disabled.  This is very useful for the user to
know for sure whether the feature got enabled or disabled.

Back in the days of buzzers, XKB was supposed to use different sounds
for each of those, but this is no longer the case now in the vast
majority of setups the beeps are intercepted and use a single recorded
sound.

XKB beeps are also unfortunately not configurable, although they
possibly should be on paper: in theory, one could alter the bell used
by listening to `XkbBellNotify` events, which provides a way to
discriminate bells through a name.  Unfortunately XKB's togglekeys
seems to suffer from a bug (?) for a long time, in that it will always
ring the `AX_IndicatorOn` bell if there is *at least one* indicator on
at the moment the bell is rung, and `AX_IndicatorOff` if and only if
*all* indicators are off.  This makes it virtually useless as it is not
possible to discriminate between an indicator getting turned on or off
in most cases, especially with NumLock which often stays always on.
Given this behavior dates at the very least as far back as X.org 1.16
which is from 2014, it probably is not very realistic to rely on a fix.

So instead implement togglekeys on our end by listening to the
`XkbIndicatorStateNotify` events.