SetGame method

Added SetGame method to GameManager.

Altered Stepmania::ChangeCurrentGame() to allow a second input,
selecting which theme will be loaded after game is changed.
SetGame lua docs
Merge branch 'SetGame' of https://github.com/sigatrev/stepmania into ChangeGame
Created GameLoop::ChangeGame analogous to GameLoop::ChangeTheme.  Changed ChangeTheme to use the AfterThemeChangeScreen metric instead of a passed in screen name from the theme being changed away from.  ChangeGame uses the AfterGameChangeScreen and AfterGameAndThemeChangeScreen metrics to pick the screen to load after the change.  New metrics are optional and default to InitialScreen.  Changed ScreenManager::ThemeChanged to not duplicate the code in ReloadOverlayScreens.  Fixed ScreenManager::IsScreenNameValid to check with HasMetric before using GetMetric.  Changed game ConfOption logic to set the preference and work in a similar way to the theme ConfOption.  Added THEME:SetTheme lua function.
Merge pull request #231 from kyzentun/ChangeGame

SetGame