client/Client: make several getter methods const

src/client/Client.cxx

@@ -82,19 +82,19 @@ Client::SetPartition(Partition &new_partition) noexcept
 }
 
 Instance &
-Client::GetInstance() noexcept
+Client::GetInstance() const noexcept
 {
 	return partition->instance;
 }
 
 playlist &
-Client::GetPlaylist() noexcept
+Client::GetPlaylist() const noexcept
 {
 	return partition->playlist;
 }
 
 PlayerControl &
-Client::GetPlayerControl() noexcept
+Client::GetPlayerControl() const noexcept
 {
 	return partition->pc;
 }

src/client/Client.hxx

@@ -230,20 +230,20 @@ public:
 	 */
 	void AllowFile(Path path_fs) const;
 
-	Partition &GetPartition() noexcept {
+	Partition &GetPartition() const noexcept {
 		return *partition;
 	}
 
 	void SetPartition(Partition &new_partition) noexcept;
 
 	gcc_pure
-	Instance &GetInstance() noexcept;
+	Instance &GetInstance() const noexcept;
 
 	gcc_pure
-	playlist &GetPlaylist() noexcept;
+	playlist &GetPlaylist() const noexcept;
 
 	gcc_pure
-	PlayerControl &GetPlayerControl() noexcept;
+	PlayerControl &GetPlayerControl() const noexcept;
 
 	/**
 	 * Wrapper for Instance::GetDatabase().