build: added maven variable for gwt css obfuscation

Added gwt.cssResourceStyle variable that sets GWT's CssResource.style
property. This property controls the GWT CSS obfuscation. Use 'obs' for
obfuscated (the default), or 'pretty' for human-readable.

'pretty' mode is very help for CSS development and debugging.

Change-Id: Icdc4b08f47a2e1570b07037ff30fc4705f308b0a
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>