Improve random video/image screensaver controls

[pjft]

Added functionality:

• Launch random screensaver from within a system, limiting the media to that system
• Add "back" and "favourites" control to screensaver, to play the previous video, and/or to add video to favourites (or currently selected collection, as if one had pressed the favourites button in the gamelist).

Changes:

• Refactored some code, namely moving the input handling code for the screensaver from the Window class to the actual SystemScreenSaver class, and auxiliary code that was there as well;
• Adjusted code for the random screensaver not to pick a new random game (for the case when you're going back, you want to feed it a game to load);
• Added option to Select menu to launch screensaver, when applicable.