More robust path detection on Windows.

[trentpolack]

HAY. I have no idea if this would be desirable, but I got tired of the project generation output saying I didn't have Houdini installed (when I do, I just never use the default path). So, I did things. Not sure where else to put this, so feel free to just close this issue if it's not useful. This also uses the SupportedPlatforms attribute on both the editor and runtime classes, so it won't build on non-Windows platforms; I wasn't immediately sure how else to make it a preprocessor check.

I updated the *Build.cs files with slightly more robust path detection using two methods to find the path:

• A manually-created (on the user scope, not system) environment variable: HOUDINI_PATH.
• Searching the registry (I'm mostly sure this works in general, but may require the user to be an administrator?) at HKEY_LOCAL_MACHINE\\SOFTWARE\\Side Effects Software.

Here's the link to the changes -- I didn't want to toss the whole file in there, so it's broken up into chunks denoted by [GIST NOTE].