Use KDE Solid for detection of USB mass storage devices

- now there is one submenu for devices reported as "HardDisk" on the USB bus, and one submenu for devices reported as one of the various "Card Reader" subtypes
- remove traces of the KDE3 media:/ ioslave

Solid is a really great thing to have, first time that I use Linux and feel the hardware is well integrated even in high-level applications!

TODO: For KDE3 we install .desktop files to add a digikam entry to the popup dialog that kded pops up when a new device is detected. I currently have no idea about the KDE4 equivalent.