Added two new widgets, GimpDeviceInfoEditor, which is an editor/view
widget for GimpDeviceInfo, and GimpDeviceEditor, which is an editor
widget for all devices. Both are pretty much ugly right now and look a
lot like the old GtkInputDialog, but are at least internally cleaned
up and easily changable code and ui wise. Consider this a completely
intermediate state.

Also cleaned up GimpDeviceInfo so it's possible to have a proper
view on it, and did the needed changes to the preferences dialog
to use the new stuff.