LustyJuggler: Key mappings are not always reset properly

Hi,

sometimes, keymappings are not reset properly (when pressing the wrong keys when LJ is open).

The first commit adds some extra cancel keys.

The second commit extracts methods for the default_cancel_keys and alpha_buffer_keys for all keyboard mappings. It then subtracts them when necessary. This way, the cancel key mappings should always be correct.

I could not get the test suite to work, though: I don't know what to do with the 'expect' command.