Mouse cursor visibility and retrospective look at updates

To be honest, I haven't got the full hang of versioning and releases yet. When I fix something, I upload it. I notice a spelling mistake, I upload it. I ought to wait for proper milestones before updating, but I'm a fledgling developer and impatient. I'll learn!

This release sees the following:

• Mouse cursor hidden during gameplay and visible in menus
• Refactored the level loading code to be simpler

Previously, updates have been:

• Implementation of lives sytem (you get 3 lives per level)
• You can now retry the level if you lost without having to go back to the very start
• Ball correctly resets to position on top of the paddle and waits to be relaunched
• Particle effects and sound effects