Calling psdk without arguments will now trigger the interactive mode.

There are a few pre-conditions:
 - empty disk available: asks if the user wants to initialize the disk
   and configure the SDK to use it.
 - persistent disk available, but not in use: asks if the user wants to
   configure the SDK to use the persistent disk.
 - persistent disk in use: no actions allowed

This patch also includes the file data/psdk.desktop which creates a
desktop icon when copied to ~/Desktop.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>