Create timer; the timer may be one-shot (it runs only once, then
triggers the alarm and removes itself), or continuous (it continues
running and triggering the alarm)
When initialised, the object will snap to grid to the nearest free
position, then it will start moving in the direction specified by
home_in, random_proportion.
Turn a (possibly) translucent or indexed image into a
display-compatible bitmask image using the given alpha threshold and
render-to-background colour.