Web Build Fixes

[SirNate0]

• Prefer the U3D shell.html rather than generating one from the emscripten default. (Fixes Web Build Slowed by Log Output #54)
• Remove the custom emscripten toolchain file. Instead, use the one installed by the emsdk, which is available by calling the cmake_emscripten script through emcmake.
• Backport a minor SDL change to setWindowTitle that is needed for newer versions of emscripten. As I have corrected it, it should be backwards compatible to old versions.
• Add a click-to-play overlay and delay running the web assembly until the user click it. Untested on mobile. Fixes Audio fails on web without user interaction #53
• Add a separate web workflow. This demonstrates how to use emcmake. Additionally, it should eventually be used to update samples on the website.

[SirNate0]

I tested the click to play on my phone and it worked fine, and I rebased onto the last change to master (I missed it earlier with having multiple remotes).

The mingw windows builds have been failing for a while now because of an issue with the action installing it (I think there is a workaround, but that can be done in a separate PR), as has the QA build.

If you want, I could disable the wasm builds within the Linux workflow, but otherwise I think it's ready to be merged.

[klaussilveira]

Let's merge it!