Make it easier to add custom cursors by extending BasicMouseHandler.

[ghost]

Hi, my name is Logan McNichols. I am an intern for Jupyter Cal Poly. I am working on a tabular data editor powered by the datagrid along with @kgoo124 and @ryuntalan.

We are working to get a grab cursor to display for moving rows and columns. The class BasicMouseHandler provides much of the core functionality we want, but it is tricky to extend it, as the cursors are determined by private properties/methods.