Textboxwidget

*    doesn't throw exceptions for macos (but doesn't work, yet)
*    removes self.region
*    uses a property to delay the creation of the cursor. You need a renderer to get a bbox, but the macos backend doesn't give you a renderer until you render the first frame. I delay creating the cursor until the first click. This makes macos happier.
*    Renamed to FloatTextBox since it is only for floating point text.
*    Inherits from AxesWidget rather than just Widget.
*    Like other mpl.widgets, it uses self.active (via self.ignore(event)) to control whether callbacks are active
*    Changed names to things i think are more descriptive.