remove gtk::Widget

This follow my previous commit, by mistake i've not made a pull request:

Here is the first commit message: 
Split widget trait
- remove Wrap / get_widget methods from Widget trait. these don't need to be public.
- move the previous methods to the traits FFIWidget, the user cannot see it.
- Widget need FFIWidget to be implemented

This ensure that a user cannot create a Widget other than one defined in rgtk as the trait FFIWidget is not public.

and the link to the commit: [split widget trait](https://github.com/jeremyletang/rgtk/commit/f985cbf10529666a68a39926521b4b920d58a305)

This second commit remove the widget struct which should not exist.