More level objects, added a new menu, bug fixes

- made the tooltips more useful and added some for objects, not just
items
- added more level objects like filing cabinets and flowers
- can now get the number of filled slots in storage units
- custom cursor icon
- tweaked the physics a bit
- added wallpaper (can be tiled)
- view discovered items by clicking the button in the crafting menu
- fixed broken recipes (wand and magnifying glass)
- cleaned up rendering of inventory slots in storage units
- added a new projectile type: fireball (wand shoots it by default)
- can now set the position on the item that projectiles spawn from
- fixed levers sometimes activating when the cursor is over them
- black holes do 1000 damage
- improved platform tiling