base-emscripten: Use EmscriptenApplication

[Squareys]

Hi @mosra!

As part of mosra/magnum#300 I thought it may make sense to also update base-emscripten. I hesitated, since desktop builds are now no longer supported. But I guess that's what the base branch is for, and this is meant as the most emscripten specific thing as far as I understand it.

This is ready once EmscriptenApplication is ready.

Cheers, Jonathan

[Squareys]

The EmscriptenApplication doc currently states:

Fully contained base application using @ref Sdl2Application for desktop build
and @ref EmscriptenApplication for Emscripten build and CMake setup is available
in `base-emscripten` branch of
[Magnum Bootstrap](https://github.com/mosra/magnum-bootstrap) repository,
...

Which could also be an option... @mosra what do you prefer?

[mosra]

The base-ios, base-android and base-winrt bootstrap projects all have both the native and the crosscompiled version, so I think it make sense to have that here as well. Besides that, Magnum design philosophy encourages people to develop natively first and then just cross-compile and having a bootstrap project that doesn't support native build would be against that.

[mosra]

Merged in f9dbbaa, thank you!