New modern default skin

Hi there,

There has been a big shift in UI design in recent years. It would be nice to see SmartClient/SmartGWT introduce a new cleaner modern theme:

* Flat look. No drop shadows, gradients, or anything like that.
* Squared corners.
* Thick borders, title bars, etc.
* Cleaner, easy to read fonts - e.g. helvetica.
* More generous padding & margins.
* Brighter (more configurable) colors.

Hopefully this can be an inclusion for SmartGWT 5.0.

-Nick