Added level-skip option

Many people have requested this feature, so here it is! You can now freely skip levels via the menu - so what are the ins and outs?
• you'll still need to collect enough droplets to enter later worlds that require droplets to unlock. This means you can't just skip your way through the whole game
• special levels can't be skipped - eg boss fights and special puzzle levels
• if you skip a level you never have to return to complete it - unless you want to search for droplets or try for a gold rank. But if you want to, you can return to complete it once the world has been completed