#18 recalculate fake transparency once in a while

[Ricardo Niederberger Cabral]

When using transparent aterms (-tr option), aterm will hang for a few
seconds while it recalculates the fake transparency whenever the
background image changed and the aterm window is activated.

This delay is annoying if your wallpaper is changed automatically
every few minutes and you have some maximized aterms. It would
be nice to have a command line option that would cause aterm to
calculate whatever it does automatically every 'n' minutes, to avoid
the annoying delay everytime you switch focus to an aterm when the
wallpaper recently changed.

Thanks.