[NbtkStyle] load the default style from the package data directory
[NbtkStyle] load the default style from the package data directory

This will allow applications to use the default style installed with nbtk.

jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx