Changes to ContentProject for Windows and debug symbols in release mode

Debug symbols in release mode are now controlled by the DebugSymbolsOnRelease property.  Previously debug symbols in release were enabled by default; they now default to off and can be re-enabled with `<Property Name="DebugSymbolsOnRelease" Value="full" />`.

Projects generated for Windows now flag their content project as `<Content>` instead of `<None>`, which has changed to assist usage with installers that automatically detect file build types.