ignore modifiers for KeyRelease bindings

ignore modifiers for KeyRelease bindings

For the following binding:

    # Simulate ctrl+v upon pressing $mod+x
    bindsym --release $mod+x exec --no-startup-id xdotool key --clearmodifiers ctrl+v

you can now use either:
1. press $mod, press x, release x, release $mod
2. press $mod, press x, release $mod, release x

fixes #485