Minor style fixes.
Add Lr cairo button for the import from Lr support.
expose the dt_iop_clipping_params_t struct.

This is needed to be able to construct a clipping item to put
in the history for the import from Lr support.
Initial implementation for the new import from Lr support.

This first implementation covers only the clipping and angle. More
automaatic import will be added later. At least this propose a
framework for the import circuitry.