Blender Git Loki

Add a new Copy As Driver context menu option for properties.

It is a very common need to create drivers that set the value of
a property to the value of some other property, but it currently
requires multiple actions: Copy Data Path on the input property,
adding a driver to the output property, selecting the input ID
reference, and pasting the path.

This adds a new Copy As Driver context menu option, which creates
a complete driver in the clipboard that reads the current property,
so all that remains is to paste it to the output property. It is
also possible to paste just the new driver variable into an existing
driver to combine multiple inputs.

Reviewers: brecht, billreynish

Differential Revision: https://developer.blender.org/D5382