Replace launcher icons by generated launcher icons

For the launcher icons I have manually created different sizes. There
are tools, for instance ADT, which do this automatically.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Place background into DPI independent folder

As the background image has to be scaled almost always, it belongs into
the DPI independent folder for drawable.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Style headline

I have changed the overall style of the headline on the main activity
layout. Among other things I have added shadows.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Style buttons

Finally I have realized a graphical concept I had in mind for the
buttons. For that I added a XML with a selector which defines the
button background images for the normal and pressed state.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>