Update CheckBox and RadioButton to use GWT API and add inline variants

First of all this PR introduces a FocusWidget with the HasId interface.

Second it adds the same API from GWT's CheckBox but allowing for two different layouts: normal and inline. I thought about controlling it via an enum but decided for two classes to allow for similiar UI XML in GWT and bootstrap: `<g:CheckBox>` vs `<b:CheckBox type="INLINE">` vs `<b:CheckBox>` and `<b:CheckBoxInline>`.

Third it turns the existing RadioButton into an actual radio input rather than a button mimicking the radio behavior. Another class should be added that implements this behavior again. RadioButtonButton?!

Fourth I added the mentioned inline classes CheckBoxInline and RadioButtonInline.

This fixes #26 and #164 (which was never reopened).