Platform: markup improvements for web applications.

Indented and cleaned up cruft from HTML template, renamed
NaClApplication.css to WebApplication.css, as it is universal and not
tied only to NaCl.
Move toolchains updating to crosscompiling documentation, clean up.
Mention availability of Application libraries on different platforms.
Updated toolchains submodule.
external: std::optional with Emscripten support.
Advertise Emscripten support.
Platform: ported Sdl2Application to Emscripten.

Emscripten supports some hybrid between SDL1 and SDL2. I don't want to
add Sdl1Application just for that and create more portability issues, so
I just changed some things in Sdl2Application to be compatible with what
Emscripten wants. Full SDL2 support is awaited in Emscripten, thus this
is future-proof (rather than having SDL1 support, which would be
deprecated later).

Added documentation about Emscripten usage and template HTML markup to
Sdl2Application docs, will move it somewhere else in the future when
more than one Application will be supported (e.g. GLUT).
Added Emscripten building documentation.
Implicitly enable BUILD_STATIC on Emscripten.