Bugfix for #198

Fixed a bug in the railgun tool. TunnelAction now cancels the activate event if it ends up destroying no blocks. I also increased its priority to HIGH so that PlaySoundAction is lower than it. The result is that an explosion sound will only play if the railgun actually destroys blocks.

See my comment in issue #198 for more detail. Let me know if you need me to change anything in the pull request.