player/Thread: move StartPlayerThread() into PlayerControl

src/command/PartitionCommands.cxx

@@ -25,7 +25,6 @@
 #include "IdleFlags.hxx"
 #include "client/Client.hxx"
 #include "client/Response.hxx"
-#include "player/Thread.hxx"
 #include "util/CharUtil.hxx"
 
 CommandResult
@@ -111,7 +110,7 @@ handle_newpartition(Client &client, Request request, Response &response)
 					ReplayGainConfig(),
 					partition.pc);
 	partition.UpdateEffectiveReplayGainMode();
-	StartPlayerThread(partition.pc);
+	partition.pc.StartThread();
 	partition.pc.LockUpdateAudio();
 
 	instance.EmitIdle(IDLE_PARTITION);