Major refactor: split codebase into multiple files

Now for the fun part: will this compile?

Sources/BigYTMiniPlayer.h

@@ -0,0 +1,8 @@
+#import "uYouPlus.h"
+
+@interface YTMainAppVideoPlayerOverlayView : UIView
+- (UIViewController *)_viewControllerForAncestor;
+@end
+
+@interface YTWatchMiniBarView : UIView
+@end