UI: rename Alpha to Opacity

There was a discrepancy between the Tooltip of the Camera's Background Image that controls the opacity of the image and the operator's name that reads Alpha. The tooltip says **"Image opacity to blend the image against the background color"**, so it was renamed to Opacity to follow the tooltip.
The decision for this change is that the naming Alpha is not the most appropriated, since it has nothing to do if a loaded image has an embedded Alpha channel or not.

{F8540101}

Reviewed By: #user_interface, pablovazquez

Differential Revision: https://developer.blender.org/D7760