Clean up field style setting.
Nuke the half-commented totalVol code.

Kills a warning.
Fix the "fillable" issue.

The code was testing for > 0 rather than >= 0. This meant that a tank could
never be empty, even in flight.
Clean up the amount/maxamount field code.

Empty fields are now allowed (interpreted as 0 when update is pressed), but
more importantly, backspace and delete can be used to clear the field.