GuiPopUpMenuCtrl fix

(This is from 1.3, don't know if it's been fixed since, but I expect not)

Under certain rare circumstances GuiPopUpMenuCtrl can get confused and open two or more pop-up menus, and not delete them when closed. While I'm fairly sure this only happens when using a mouseThrough modification, such modification is quite common. Plus, this fix is very simple.

Add the following line at the top of void GuiPopUpMenuCtrl::onAction():

void GuiPopUpMenuCtrl::onAction()
{

   // new line
   closePopUp();

   // old stuff
   GuiControl *canCtrl = getParent();
   ...