Rename widget class & add custom tax widget class files

We are going to create two custom taxonomy widgets, to work around the inflexibility of the built-in categories widget.
Both of these new widgets will use WP_Widget_Categories as a base, which we have copied into their respective class files.
Add and include custom taxonomy widgets

Can't currently make them work in dropdown mode.
Given that dropdown mode hasn't been asked for, it's just not going to be enabled this time around :)