Multipleselection avoid triggering mouseDown event when clicking inside the selected area

[brauliodiez]

Right now we have multiple selection.

But we can drag all the selection if we click over a selected shape if we click over the white space inside the selection it will trigger the drag and drop multiple selection.

Whenever a mousedown event is triggered on the stage we have to set a temporary flag of darggingInsideMultipleSelection to avoid this behavior