Make zoom widget make sense

[diane]

The current calculation for the zoom widget is rather odd. It'd be better if the spin box value equaled (base pair/pixel)*10. (That way 1 would be 1 base pair for 10 pixels or fully zoomed out).

Also there should be a "zoom fully out" option which would then set the zoom size to show whatever our current maximum world coordinates are.

(This'd also help the aligned connection view (ticket:4) code as to show the equivalent to the old mussa sequence view I could just set the zoom level to something around 1..5. (aka 1 bp for ever 5 to 10 pixels.)