naive implementation of 'bindsym --release' (and bindcode)

naive implementation of 'bindsym --release' (and bindcode)

The implementation is naive because the user has to generate exactly the
event he specified. That is, if you use this binding:

    bindsym --release $mod+x exec import /tmp/latest-screenshot.png

Then it will only be triggered if you hit $mod, hit x, release x,
release $mod. It will not be triggered if you hit $mod, hit x, release
$mod, release x. The reason is that the KeyRelease event in the latter
case will not have the modifier in its flags, so it doesn’t match the
configured binding.