Merge pull request #2343 from SickheadGames/PS4PlatformEnum

Add PlayStation4 to content pipeline target platforms.
Use single PlatformClear method and clear non-applicable options.
Merge pull request #2335 from jbeshir/fix-clear-options

Fix bug where GraphicsDevice.Clear() always cleared stencil and depth buffers.
More XmlImporter functionality.
Adding XML asset test case data and matching classes.
3.2 update of the NuGet packages
Now including iOS / Android / MacOS for the binaries package
Will update template NuGet for these platforms soon
Removed additional SDL.Dll and sourced from "ThirdParty\GamepadConfig"
Update NuSpec to source from this location
BasicMaterialContent now uses opaque data to all its properties.
Made texture dictionary readonly.
Use texture setter for basic material.
Added missing MaterialContent classes.
Merge pull request #2355 from SickheadGames/matfix

MaterialContent Fixes
Trying to fix iOS build triggering.
Fixed malformed if test.
Disabled Video.BackgroundColor to make iOS build.
Fixed PlatformResume() for iOS.
Merge pull request #2356 from SickheadGames/iosfix

Fix iOS Build
Added support for bypassing the content pipeline in MGCB
Added missing SurfaceFormats.
Expose PathHelper to MGCB and use its path normalization instead.