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