Remove LauncherSettings

Icon size and alignment all depend on the containment, the only setting
we need to load and save is the list of favorite applications.
We can use QStaticConfiguration for this, hence we can do some spring
cleaning and remove LauncherSettings.

Issue: #148
Remove ShellSettings

We can use QStaticConfiguration from C++ and the Configuration item
from QML, there's no need for ShellSettings anymore.

Issue: #148
Remove settings QML plugin

Issue: #148
desktop: Load background immediately the first time

The animation takes some time thus the user see a flash when loading
for the first time.  To avoid that we set opacity without the animation.