Make the webapp file system aware

[freezy]

Lots of cool stuff could be done by giving the browser access to the file system, such as:

• Tag locally available files when browsing
• Filter by local availability
• Download directly to the right place instead of a .zip file
• Make it easier to upload stuff (faster selection, grab available media)
• Update PinballX.xml after download

For reading and parsing it's probably a good idea to use a web worker.