Efl.Interpolator*: More sensible API names and docs

Efl.Interpolator*: More sensible API names and docs

Summary:
OK, so, ALL interpolator parameters were called "factor" and the docs
literally said "First factor, Second factor, ..."
After diving into the actual implementation, proper names (and types) for the
parameters were found and proper docs written.

I am afraid I could not make any sense of the Divisor interpolator code. Those
docs still need writing.

Test Plan: Everything still builds and passes tests. No functional changes.

Reviewers: zmike, cedric, bu5hm4n, Jaehyun_Cho

Reviewed By: bu5hm4n

Subscribers: #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D10603