Add small text line with key shortcuts to Song Screen

Add a small text line with key shortcuts to Song Screen, toggled if the search bar is active with help to the extended search keywords added by another commit.

Vocaluxe/Screens/CScreenSong.cs

@@ -42,7 +42,7 @@ namespace Vocaluxe.Screens
         // Version number for theme files. Increment it, if you've changed something on the theme files!
         protected override int _ScreenVersion
         {
-            get { return 6; }
+            get { return 7; }
         }
 
         private const string _TextCategory = "TextCategory";