Don't load multiple version of the same add-on

Now solely the one of highest priority is used (e.g.,
`non-packaged/add-ons/` over `system/add-ons/`).

Fixes #20

application/ChatProtocolAddOn.cpp

@@ -27,6 +27,13 @@ ChatProtocolAddOn::ChatProtocolAddOn(image_id image, const char* path, int32 sub
 }
 
 
+ChatProtocolAddOn::~ChatProtocolAddOn()
+{
+	delete fIcon;
+	unload_add_on(fImage);
+}
+
+
 status_t
 ChatProtocolAddOn::InitCheck() const
 {

application/ChatProtocolAddOn.h

@@ -17,6 +17,7 @@ class ChatProtocolAddOn {
 public:
 					ChatProtocolAddOn(image_id image, const char* path,
 						int32 subProto=0);
+					~ChatProtocolAddOn();
 
 	status_t		InitCheck() const;
 

application/ProtocolManager.cpp

@@ -47,10 +47,18 @@ ProtocolManager::Init(BDirectory dir, BHandler* target)
 		if (id < 0)
 			continue;
 
-		// If add-on's API version fits then load accounts…
+		// Refuse to load add-on under some circumstances…
 		ChatProtocolAddOn* addOn = new ChatProtocolAddOn(id, path.Path());
-		if (addOn->Version() != APP_VERSION)
+		if (addOn->Version() != APP_VERSION || ProtocolAddOn(addOn->Signature()) != NULL) {
+			if (addOn->Version() != APP_VERSION)
+				printf("%s not loaded, due to insufficient version (%i v %i).\n",
+					addOn->Signature(), addOn->Version(), APP_VERSION);
+			else if (ProtocolAddOn(addOn->Signature()) != NULL)
+				printf("%s not loaded, due to another instance already having been loaded.\n",
+					addOn->Signature());
+			delete addOn;
 			continue;
+		}
 		ret = true;
 
 		// If add-on has multiple protocols, also load them