*Make 'allowedthings' default to previous level's one.
*fix lev6 help
*bugfix: whitecats aren't ever appearing randomly?
*new object: bricks
*Don't allow extending path off the top of the grid.
*fix incorrect pluralisation "foods" in goal text
*help text for all existing levels
*Store level progress and hiscores using localstorage
*new thing type: goats
*goat help text.
*level select screen needs "back to title"
*mechanism to force at least one of a given thing type on a level.
*better level complete animation - zoom in random cat picture.
*only redraw screen if dirty!
*switch to "star" system for unlocking levels
*fix bug when calculating whether there are any valid moves left
*fix bug in random thing selection
Rob Pearce