QR code scanner fixes

A few fixes for the QR Code scanner plugin:

 * First commit refactors the API to conform with 1.9. Mostly fixes of self.gui -> self.gui.main_window. Moves the send tab gui modification functionality into a load_wallet event hook so as to ensure that the wallet is loaded beforehand.
 * Second commit adds a settings dialog to be able to select the video device to use in the case that the user has multiple cameras installed on their system. On Linux-like machines with /sys access, the list of available devices is pre-populated. On other systems, the user must enter the device name manually if they want to use anything but the default.