* #6226 * #6179 : Rework v4l2 classes / preferences

VideoV4l2List doesn't store the current settings anymore
Preferences are actually saved and restored
Properties (channel, size, rate) are all stored as strings
remove VideoV4l2Rate (it was only a float)

VideoV4l2List will have a thread monitoring devices insertion/removal with udev and will notify the preferences code in the daemon, and the client

Fix bug in VideoV4l2Channel::GetSizes() : mix class member and local variable (idx)

Print error messages in client dbus functions
Return NULL strings in client dbus functions in case of error, instead of uninitialized strings