Improved trait implementation macros. Implemented positioning helper methods. Updated readme.

- The macros have cleaned up the widget modules slightly!
- The positioning helper methods are demonstrated in the `all_widgets` example. Instead of having to do this `.position(x, y)` you can now just do `.down(padding)` to place relatively to the last widget. You can also place relatively from any other previously drawn widget i.e. `.right_from(uiid_of_other_widget, padding)`
- README.md is now up to date with refined goals, new available widgets and 'current state'.