Added tabbar adding event

Fired when the add button of the tabbar is pressed.
Changing the value of arg.add to false the action can be vetoed.
The value arg.where indicates the position of the new item.

source/gui/widgets/tabbar.cpp

@@ -751,15 +751,27 @@ namespace nana
 					if((pos == npos) || (pos >= list_.size()))
 					{
 						pos = list_.size();
+						
+						if(evt_agent_)
+							if(!evt_agent_->adding(pos))
+								return false;
+
 						this->list_.emplace_back();
 					}
 					else
+					{
+						if(evt_agent_)
+							if(!evt_agent_->adding(pos))
+								return false;
+
 						list_.emplace(iterator_at(pos));
+					}
 
 					basis_.active = pos;
 					if(evt_agent_)
 					{
 						evt_agent_->added(pos);
+						erase(pos);
 						evt_agent_->activated(pos);
 					}
 					return true;