JS Bindings: Conveniently bind functions with optional parameters

[frericp]

This PR introduces a way to conveniently create bindings for functions with optional parameters. The following compares the old with the new approach:

Old: Create a single binding that expects all parameters, including optional ones (due to a limitation of Emscripten). Allow to not provide optional parameters by adding additional logic on the JavaScript side, which essentially fills in all non-provided optional parameters.

New: Use macros to automatically register multiple bindings for a function, each taking a different number of parameters. This has several advantages:

• Compatibility of bindings with the actual source code is asserted at compile time (except for the cases where a previously required parameter becomes optional, or an additional optional parameter is added to the end of a function's declaration).
• Bindings don't need to be maintained in two different places (JS and C++) anymore.

All functions of the Document class that had default parameters defined in the JS boilerplate code have been redefined for illustration purposes.

Furthermore, this PR fixes an issue with the JS boilerplate code, which broke the function overloading capabilities of Emscripten.

FYI @sdunkel @kohakukun

[frericp]

That's funny, because I explicitly renamed it from BIND_FUNC to BIND_CLASS_FUNC before, since it only works on class member functions (i.e. you have to provide a class parameter to the macro). But I wasn't aware of the emscripten::class_function. Will change it back...