Remove jsevents object

[juj]

This pull request removes the JSEvents object altogether in src/library_html5.js and replaces it with the use of explict x__deps dependency handling. This allows dead code elimination to remove a lot of unused functions when using the HTML5 events API. Also the functions are now prefixed with double underscores to clearly separate public and private APIs. This fixes #2999.

[juj]

The intent of this is also to prepare for fixing issue #3283 by changing the built-in default shell.html to reuse the HTML5 functions so that the fullscreen button and the HTML5 API fullscreen request code will play nice together. However I'm not sure yet how to model a dependency between default shell.html to _emscripten_request_fullscreen. Would you have an idea for this @kripken ?

[kripken]

I think if an HTML shell file depends on something, it would need to add it as used by adding it to DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.

[kripken]

This is a lot of refactoring that leads to what I worry is worse code - now there will be lots of small globals (_currentEventHandler, _eventHandlers, etc.) as well as functions. It's all flattened out in the global namespace, with no obvious structure, unlike now. Is this really worth it to allow dependency management to remove some of this code?

If we are sure that the code size benefits are worth it, how about if there is still a $JSEvents, but it starts out almost empty, and at runtime we add properties to it when needed? Not sure if that would work or not.

Another option is to rely on closure. Closure advanced can remove parts of objects, so it would do dead code elimination in the code before this pull.

Another option is to add dependency management in emscripten to properties of objects, but that sounds complex.

[kripken]

Is this still important, @juj?

[juj]

This is not only important, but critical; though this PR that implements this removal has fallen out of date, so I'll close this out. In my fork november_2018 I've reimplemented this, but don't have time just now to submit a PR.