Transfer drivers on lights from 2.7x to 2.8

Since the light types have changed from Blender 2.79 to 2.8, the intensity of the light is no longer controlled by an emission node in a shader node tree but by a new "energy" value. If you now open an 2.7x file in 2.8 and have a driver controlling the lights intensity (on the emission node vie the "strength" field), this driver is ignored and the "energy" field is set to a default value. It would be useful to transfer the value of the "strength" field in emission node to the new energy field when opening the 2.7x file in 2.8. If the emission node contains a driver, it is copied as well.