handle height of sorting window properly

before, if terminal was less than 18 lines high, ncmpcpp
would exit at start throwing NCurses::Window::BadSize,
because sort dialog's height was hardcoded. now its size
changes dynamically, depending on actual screen size.

src/playlist.h

@@ -93,7 +93,7 @@ class Playlist : public Screen< Menu<MPD::Song> >
 		
 		static const size_t SortOptions;
 		static const size_t SortDialogWidth;
-		static const size_t SortDialogHeight;
+		static size_t SortDialogHeight;
 };
 
 extern Playlist *myPlaylist;