Add silent keep/discard inventory offer buttons

Adapted from Phoenix code

indra/newview/llviewermessage.cpp

@@ -1603,6 +1603,31 @@ bool LLOfferInfo::inventory_offer_callback(const LLSD& notification, const LLSD&
 		}	// end switch (mIM)
 		break;
 
+	case -2: // decline silently
+	{
+		log_message = "You silently decline " + mDesc + " from " + mFromName + ".";
+		chat.mText = log_message;
+		LLFloaterChat::addChatHistory(chat);
+	}
+	break;
+	case -1: // accept silently
+	{
+		LLOpenAgentOffer* open_agent_offer = new LLOpenAgentOffer(mObjectID, from_string);
+		open_agent_offer->startFetch();
+		if(catp || (itemp && itemp->isFinished()))
+		{
+			open_agent_offer->done();
+		}
+		else
+		{
+			opener = open_agent_offer;
+		}
+		log_message = "You silently accept " + mDesc + " from " + mFromName + ".";
+		chat.mText = log_message;
+		LLFloaterChat::addChatHistory(chat);
+	}
+	break;
+	
 	case IOR_BUSY:
 		//Busy falls through to decline.  Says to make busy message.
 		busy=TRUE;

indra/newview/skins/default/xui/en-us/notifications.xml

@@ -6721,7 +6721,25 @@ An object named [OBJECTFROMNAME] owned by (an unknown user) has given you a [OBJ
    name="UserGiveItem"
    type="notify">
 [NAME] has given you a [OBJECTTYPE] named '[OBJECTNAME]'.
+
+(Keep)/(Discard) won't send an accepted/declined message.
     <form name="form">
+      <button
+       index="2"
+       name="Mute"
+       text="Mute"/>
+      <button
+       index="-1"
+       name="(Keep)"
+       text="(Keep)"/>
+      <button
+       index="-2"
+       name="(Discard)"
+       text="(Discard)"/>
+      <button
+       index="4"
+       name="Profile"
+       text="Profile"/>
       <button
        index="0"
        name="Keep"
@@ -6730,14 +6748,6 @@ An object named [OBJECTFROMNAME] owned by (an unknown user) has given you a [OBJ
        index="1"
        name="Discard"
        text="Discard"/>
-      <button
-       index="2"
-       name="Mute"
-       text="Mute"/>
-      <button
-       index="4"
-       name="Profile"
-       text="Profile"/>
     </form>
   </notification>