Update artwork for logout dialog:
New Oxygen design

The sourcecode has no new features. I have only changed coordinates, colors and such things.

Due to render problems in Qt, we use a 'pre'-rendered svg image (exported as png and imported again into the svg).

The real source svg is also included: shutdowndialog.source.svg

I have also moved the button svgs into the main theme svg (with a different object id).

This dialog needs some more love after feature freeze is over:
* do not hardcode fonts and colors
* improve strings
* probably change buttons layout
* ...

For informations of the moon image used, see the CREDITS file (photo released under a Creative Commons license).

I think this file should be enough; if not, please tell me.

Thanks a lot Pinheiro for the great work!