upgrade to 2.7-beta1

Notes on moving from external GssResource to internal CssResource using Gss:
- Bump gwt version to 2.7-beta1
- Remove GssResource and related pom dependnecies 
- Revert resource interfaces from 'extends GssResource' back to 'extends CssResource'
- UiBinder: remove the type definition in <ui:style type="...">
- Remove: <inherits name="com.google.gwt.resources.GssResource"/> in the gwt module
- Add the three config flags

Note: If you were not previously using GssResource from the external jar:
- Just add the flags (???)

```
<!-- Enable Gss with CssResource -->
<set-configuration-property name="CssResource.enableGss" value="true" />
<set-configuration-property name="CssResource.legacy" value="true" />
<set-configuration-property name="CssResource.conversionMode" value="strict" />
```