Remove unneeded "updatePreview" code from certain functions

The preview will automatically be drawn by the command bar, so we can
load these forms a bit quicker by not requesting separate preview
updates.
Store curve tool values as relative values (rather than absolute)

Fixes an extremely rare bug where the user changes monitor DPI settings
to a lower value, then loads a PD instance with saved curve values.
Because of the lower DPI, the old curve values may now lay outside the
curve box, which causes problems with the cubic spline calculation.