Nova v0.18-alpha.6

• Added Canvas gui element
• Fixed #535 - Nova is causing strings and tripwires to be unbreakable
• Fixed an issue where lines of flying leaves would appear in the world
• Fixed an issue where leaves, note blocks and tripwire would not have block break effects
• Fixed an issue where leaves, not blocks and tripwire could not be picked via the middle mouse button
• Fixed an issue where side configuration menus would not be closed when the associated tile entity was destroyed
• Fixed an issue where some tile entity guis would stop updating after switching menus