add config options for communication mode with mpd (polling for default)

idle support in ncmpcpp is marked experimental since its support
in mpd is not ready for general use in full featured clients
(see bug 2612)

src/mpdpp.h

@@ -94,6 +94,7 @@ namespace MPD
 			
 			float Version() const;
 			
+			void SetIdleEnabled(bool val) { isIdleEnabled = val; }
 			bool SupportsIdle() const { return supportsIdle; }
 			void OrderDataFetching() { hasData = 1; }
 			int GetFD() const { return itsFD; }
@@ -223,6 +224,7 @@ namespace MPD
 			
 			int itsFD;
 			bool isIdle;
+			bool isIdleEnabled;
 			bool supportsIdle;
 			bool hasData;