Bug 731765 - "Value Propagate" dialog's "Propagating rate" spinbuttons...

...are much too quick

In gimp_scale_entry_new(), set the spinbutton's climb_rate to the same
value as step_increment instead of hardcoding 1.0. This should make
many spinbuttons behave better.

(cherry picked from commit b9265ebfb9166d2819d21410740c44ffad0f1682)