Version 0.5.2

The rules check runs about twice as fast, and saving a level now checks it in the background

• The rules check is 1.9 to 2.4 times faster. On the volcano test level (19 341 objects) it takes 850 ms instead of 1 614 ms, on weathergirl (4 151 objects) 112 ms instead of 264 ms. The editor runs it when it opens a level, so large levels open faster
• Saving a level checks it against the rules in the background, after the file is already on disk, and reports how many findings there are. Nothing is fixed automatically
• On the Rules tab each row is coloured by its severity: error, warning or advice