Implement partial dictionary codegen support

[jdm]

#3585 adds a commented out partial dictionary definition to KeyboardEvent.webidl. I doubt that this is a particularly big deal to port from Gecko's CodeGen.py (possibly requiring porting changes from parser/WebIDL.py as well), but I don't have much to go on. This could be a fun task for someone who'd like to learn more about the DOM binding code generation.

[ChrisParis]

It looks like Gecko's WebIDL parser and Codegen also do not support partial dictionaries. Dictionaries defined in parts in the spec are combined into single definitions. For example, http://mxr.mozilla.org/mozilla-central/source/dom/webidl/KeyboardEvent.webidl.

[jdm]

In that case that's definitely not a "less easy" task to implement :)