Added Keyboard Shortcut for Most Recently Played Game

This is similar to [Pull Request #1250](https://github.com/OpenEmu/OpenEmu/pull/1250), except it adds a shortcut key for only the most recently played game. In addition, rather than assigning weird `Command+Shift+A,B,C,D,E` type keyboard shortcuts, this update uses the much more logical `Command+R`. This fits well into the UI, as `Shift+R` takes the user to recently added games, thus adding a degree of UI consistency that seems "natural" to users.