Added simple composable to store the dragged item.

Reasoning: Be able to determine what type of item is being dragged over. With this information I can determine if item can be dropped.

Example: "Column" is being dragged over "Task". We want to forbidden dropping the Column on the Task. Column needs to be dropped on a Column.

Implementation note:

• Custom transfer added to event.dataTransfer on dragstart is not accessible on dragover | dragenter | dragend | dragleave | drag. It is accessible only on dragstart | drop events.
• For this reason I decided to store item in the Composable ref.

Final note: I am aware the solution is not perfect. We could use only created Composable and ditch using event.dataTransfer at all. However the purpose of working on this project was to learn more about "drag and drop" Browser feature. This repo remains a reference to this learning process and that is why I am going to keep this "mixed" implementation of both.