Item icon imports and inventory system is working

The player inventory system is working, and the server is now sending
information to the client whenever the client's inventory changes. The
system is still very fragile, so will be working on it more.

Also need to get dragging and dropping items working. I'm very close but
there's some more work to be done.

This commit also accidentally includes support for LRU blocks in the map
editor, but not on the server-side yet. However, all maps have been
updated to export the "block type" information for blocks on the map.