Custom cursors for EllipseSelect and RectangleSelect tools

User IgorZ mentioned this as a usability bug in our Google group and I agree with him. The LassoSelect tool has a custom cursor, but the RectangleSelect and EllipseSelect tools don't have one and use the default OS mouse pointer instead. I added the other two cursors for the sake of consistency. If you agree with this change, please merge it in the main Pinta repository. If you prefer to have these kind of changes discussed beforehand, please let me know. I'm still new to development on open-source software. Thanks!