GroupBase incorrectly computes hasMouseListeners which breaks e.g. mouseEnabledWhereTransparent

The `GroupBase#hasMouseListeners` property is vital for things like dispatching mouse events on transparent parts on the group, however, the current algo is incorrect - it just increments a counter every time addEventListener() is called and decrements it when removeEventListener() is called.

That means that when I call addEventListener("click") and then removeEventListener("someOtherMouseEvent"), the hasMouseListeners property will be false even though I have an active "click" listener.