Improve drag & drop from notebook

[fcollonval]

This PR fixes partly #87:

• When dropping a cell from a notebook, the widget size is set according to the cell size
• Add button to compact the grid on the top left corner
• Add helper text when there are no widgets in the editor
• Block dragging early - don't wait for the drop to tell the user a cell is not a valid source.

Failed to insert a virtual widget to help placing the cell when drag & dropping it from the notebook

[hbcarlos]

Thanks, looks great!

[hbcarlos]

Btw, did you try to update to gridstack v4.0.1 here? What happens, is not working fine?

[fcollonval]

Did you try to update to gridstack v4.0.1 here? What happens, is not working fine?

From my tests, I did not encounter troubles - but I wanted to focus on one feature by PR (what I don't do enough 😉 ).

[hbcarlos]

No! I really appreciate these PRs, this is a huge improvement!