Convert POI handling code to Typescript and add error check and null pointer yak shave

[ZeLonewolf]

This PR:

1. Converts the POI handling code to typescript and adds typing in a couple places to help with editor code completion
2. Adds a check to make sure the supplied POI icon name actually exists. This was a problem I actually ran into when I typo'd a POI icon name and had to follow a stack trace to figure out the problem. With this change you'll get a helpful console error message.
3. Adds a check to make sure a webfont shield is really missing from the spritesheet before trying to remove and add it. This fixes a race condition which caused a bunch of errors to show up in the console on start.
4. Fix for this occasional startup stack trace. It occurs in the case where the shieldJSON hasn't loaded yet, but the tiles have, and it's starting to render shields before we have the shield definition (all the more reason why we should do Compact ShieldJSON format #1015)