Update the copyrights to include full name attribution to all
who contributed to the clock watch face, including myself.
Also add an SPDX license identifier header comment to the files.
Instances of the clock state structure
are only passed to the clock face itself
and only via the opaque context pointer.
No other code uses it.
Thus there is no need to expose it in a header file.
So make it an implementation detail of the watch face
by localizing it inside the translation unit.
