Make XMB more efficient

[Philip]

Do various things like:

• Store strings as UTF-8
• Add GetValueAsFloat() etc to the API
• Store typed data, for more efficient int/float/etc storage. (And have something in the API to define which storage type to use for a given element/attribute.)
• Store repeated strings (e.g. entity names in maps) once in a table, instead of storing them repeatedly.