Introduce debug setting, LiruNewMessageSoundIMsOn, for defaulting ding of new IMs on.

This setting is off by default and I don't intend to give it a gui yet.
It probably will go in Chat UI, eventually.

indra/newview/app_settings/settings.xml

@@ -939,6 +939,19 @@
       <key>IsCOA</key>
       <integer>1</integer>
     </map>
+    <key>LiruNewMessageSoundIMsOn</key>
+    <map>
+      <key>Comment</key>
+      <string>Whether or not ding defaults to being on for new IMs.</string>
+      <key>Persist</key>
+      <integer>1</integer>
+      <key>Type</key>
+      <string>Boolean</string>
+      <key>Value</key>
+      <boolean>0</boolean>
+      <key>IsCOA</key>
+      <integer>1</integer>
+    </map>
     <key>LiruNoTransactionClutter</key>
     <map>
       <key>Comment</key>

indra/newview/llimpanel.cpp

@@ -347,6 +347,7 @@ LLFloaterIMPanel::LLFloaterIMPanel(
 	case IM_SESSION_P2P_INVITE:
 		mVoiceChannel = new LLVoiceChannelP2P(mSessionUUID, mLogLabel, mOtherParticipantUUID);
 		LLAvatarTracker::instance().addParticularFriendObserver(mOtherParticipantUUID, this);
+		mDing = gSavedSettings.getBOOL("LiruNewMessageSoundIMsOn");
 		break;
 	default:
 		llwarns << "Unknown session type" << llendl;