Couple of small fixes in GWT backend

And therefore combined into one pull request. I hope this is ok.

* added log configuration with GwtApplicationConfiguration.log
* more accurate value of GwtGraphics.getDensity()
* Skin emulation takes NinePatch padding into account (copy-paste from not emulated class)
* fixed filtering of asset files in PreloaderBundleGenerator

I also faced another issue, which is not included in the patch:

The style declaration

    <inherits name='com.google.gwt.user.theme.chrome.Chrome'/>

in [gdx_backends_gwt.gwt.xml](https://github.com/libgdx/libgdx/blob/master/backends/gdx-backends-gwt/src/com/badlogic/gdx/backends/gdx_backends_gwt.gwt.xml) is quite convenient, but if the application is embedded in a web page and the gwt module is loaded in the end, then this style overwrites page styles. To get rid of it is not easy (in fact I was not able to turn it off and use modified jar files). Maybe it makes sense to move this style from the library module to user application modules to allow better theme customization.