More work done on the form system, but the cleanup isn't quite finished yet.
Moved support for choices (whitelist validation + <datalist> autocompletion) and multiple values into the base form `Field` class and started deprecating field classes specific to choices or multi support.

The endgame should™ be a cleaner *and* more flexible form system.
 
Also, there are now template hints shown when in debug mode, because I finally got tired of having trouble finding the right template file for some output: