Update the installer look and feel from SLE12 (bnc#874153)

Copied from
https://github.com/yast/yast-theme/tree/f6f21b07b8e516e896141e63ace310efecd4b866/SLE/wizard
Make installation_slim.qss the same as installation.qss (bnc#874153)

it is questionable whether they should *stay* the same:
- there is no big image on te background anymore, which prompted the
  split to save network bandwidth (bnc#742777)
- some logos have been added
Merge pull request #44 from mvidner/installer-look-update-from-sle12

Installer look update from sle12 (bnc#874153)