kiosk-shell: Read background-color from ini file

desktop-shell's client is able to read-up from the config file, [shell]
section the background, but for kiosk-shell we don't actually have
client that does that, so instead allow the shell do that directly.
Seems to be a useful thing to have, as a default background color.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>