Client: replace playlist and player_control with getter methods

Prepare to convert "partition" to a mutable pointer.

src/client/Client.hxx

@@ -41,6 +41,8 @@ class SocketAddress;
 class EventLoop;
 class Path;
 struct Partition;
+struct PlayerControl;
+struct playlist;
 class Database;
 class Storage;
 
@@ -49,8 +51,6 @@ class Client final
 	  public boost::intrusive::list_base_hook<boost::intrusive::link_mode<boost::intrusive::normal_link>> {
 public:
 	Partition &partition;
-	struct playlist &playlist;
-	struct PlayerControl &player_control;
 
 	unsigned permission;
 
@@ -187,6 +187,12 @@ public:
 	 */
 	void AllowFile(Path path_fs) const;
 
+	gcc_pure
+	playlist &GetPlaylist();
+
+	gcc_pure
+	PlayerControl &GetPlayerControl();
+
 	/**
 	 * Wrapper for Instance::GetDatabase().
 	 */