New wildcard expansion mechanism

Hi, 

I was not happy with the wildcard expansion mechanism as it did not allow me to organize my files as I wanted to. Therefore, I've reimplemented it in a cleaner and more flexible way which should be easier to manage in the future. The reason why it did this is that now also 'wildcard modifiers' are supported (see commit message for description and examples). 

The french error messages are still missing (marked by TODO in the zotero.properties file) and there are a few design decisions to make for you (marked by TODO inside zotfile.js). However, the code runs as is so you can try it out! 

Maybe this is overkill for the average "non-computer-affine" user, but in this case one could hide the most commonly used functionality behind options and still leave the modifiers there for the experts. Please tell me what you think about this.

Best,

Simon

PS: If you're not happy with the symbols I chose, change them to your liking.