Rename groupmaster to groupleader and related variables. (#1875)

Fix the naming conventions. 
Master should be reserved to identify a bots Master. 
groupleaders are not necessarily group masters and it should be clear
what the bot is looking for. (In most solo cases leader=master)

src/strategy/actions/TellMasterAction.cpp

@@ -22,7 +22,7 @@ bool OutOfReactRangeAction::Execute(Event event)
 
 bool OutOfReactRangeAction::isUseful()
 {
-    bool canFollow = Follow(AI_VALUE(Unit*, "master target"));
+    bool canFollow = Follow(AI_VALUE(Unit*, "group leader"));
     if (!canFollow)
     {
         return false;