Improved Singleton classes instantiation

Due to BGSprite being instantiated in Title screen, Game1.cs had a
quirky requirement where Title screen must be initialized before any
other screen. This requirement was unclear, and no errors were thrown if
another screen attempted to use global scrolling background and was
initialized prior to Title - the background just did not appear on that
screen. Converted fields to properties (consistent with .NET Singleton
pattern), and added exceptions to all public methods to prevent
ambiguities in code. Moved creation of BGSprite singleton to Game1
class.

NOTE: Current implementation of BGSprite and GameContent singletons is
not thread-safe.