This is the first in a series of commits to refactor the color handling
in sway. This changes parse_color to return whether it was success and
no longer uses 0xFFFFFFFF as the fallback color. This also verifies that
the string actually contains a valid hexadecimal number along with
the length checks.
In the process of altering the calls to parse_color, I also took the
opportunity to heavily refactor swaybar's ipc_parse_colors function.
This allowed for several lines of duplicated code to be removed.
97f9f0b699