Add support for autodetection of gphoto2 cameras with Solid.
As for USM cameras, all other cameras supported by Solid will now be detected dynamically and appear in the menu when they are connected.

The cameras added manually are moved to their own submenu.
Adding manually _should_ now only be necessary for serial port devices.

Note 1: This needs testing.
Note 2: I am not sure about the right solution when no devices are connected:
Should the submenu be hidden, disabled (current solution) or one entry be added "No device connected"?
Note 3: This requires kdelibs at least revision 704641, later than last monday (27.8.)
A necessary method in Solid was added only then.