Add blank icon as default image

Empty dconf is causing some disturbnce in the layout. The same could
happen if there is no oled-label defined for a button Adding blank.png
as the default image solves the problem.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Update TODO

Signed-off-by: Przemo Firszt <przemo@firszt.eu>