Improve metadata format

Use camel case entry names like metadata for Qt plugins.

Issue: #148
desktop: Load background plugins synchronously
Use more appropriate licenses

Use GPL2+ for executables and LGPL2.1+ for libraries.

Issue: #148
server: Move screen saver code to a class

Issue: #131
server: Move notifications to its class

Issue: #131
server: Move layers where they belongs

Move notifications layer to the notifications interface, other layers
are moved to DesktopShell.

Issue: #131