friendchatmanager: rename to chaticonmanager

FriendsChatManager predates the api having an ability to look up clan
members by name, without doing string matching, hence the caching. This
is no longer necessary as we can do efficient lookups of players by name
now.

Also remove the old isMember utility function which is just duplicated
from the friends chat manager.

runelite-api/src/main/java/net/runelite/api/EnumID.java

@@ -34,4 +34,5 @@ public final class EnumID
 	public static final int MUSIC_TRACK_NAMES = 812;
 	public static final int MUSIC_TRACK_IDS = 819;
 	public static final int XPDROP_COLORS = 1169;
+	public static final int FRIENDS_CHAT_RANK_ICONS = 1543;
 }