Update: Add missing header and add/update static variables relating to strings.

Update: Add missing header and add/update static variables relating to strings.

The string.h header is not being directly included by f_color and should be.
Also add the missing dependency.

This adds a small amount of static variables.
There is more work to do in this regard, such as with f_file related strings.

Replace fll_error_string_null_s with f_color_set_string_null_s.
Utilize f_string_empty_s in place of "".

This skips making changes in controller.
The appropriate changes will be made in the appropriate work in Progress commit.