Implement DragAndDropContainer::setShouldCheckExternalDragContinuously()

Currently a DragAndDropContainer only calls shouldDropFilesWhenDraggedExternally() once when the drag operation leaves the JUCE window.

This precludes the possibility allowing drop based on some condition that changes during the external drag. For example, when the user drags a file some process may start (e.g. converting and writing to disk) and we may want to check if this is complete during the external drag, only returning true from DragAndDropContainer::shouldDropFilesWhenDraggedExternally() when that process is complete.

Adding a method setShouldCheckExternalDragContinuously() to DragAndDropContainer would allow the following change in DragAndDropContainer::checkForExternalDrag()

void checkForExternalDrag (DragAndDropTarget::SourceDetails& details, Point< int > screenPos)

{

if (! hasCheckedForExternalDrag || shouldCheckExternalDragContinuously)
{
      // check for drag and call owner.shouldDropFilesWhenDraggedExternally()