msbuild: Add property for creating Steam builds on Windows

[OatmealDome]

Forgot that Windows doesn't use CMake still. Specify /p:Steam=true with msbuild to create a Steam build for Windows.

I've also added a build flag to disable the auto updater. Use /p:AutoUpdate=false to disable the auto updater functionality.

[shuffle2]

I’m clueless about steam. What is the point of this? Is it required to submit the app to steam store?

Please rebase and put the flags in this file https://github.com/dolphin-emu/dolphin/blob/master/Source/VSProps/Base.Dolphin.props

[OatmealDome]

@shuffle2 Rebased.

We might want to introduce Steam-specific features in the future, and since those won't work when running outside of the Steam environment, they should be locked behind a #ifdef STEAM. Steam will also handle updating for us, so we should disable our own autoupdate infrastructure since it'll interfere and be confusing for the user.

[shuffle2]

Does it really need compile-time difference? I suppose it doesn't really matter, but seems code might be cleaner / more robust if it's just runtime-dynamic. anyway this pr is fine with me.