QEasingCurve: The setting order of properties should not matter.

Previously, this failed because QEasingCurveFunction only had a linear
behavior.
The fix is to change that and let QEasingCurveFunction handle
any of the simple "non-parametric easing" functions.

Task-number: QTBUG-38686
Change-Id: I666d59e10ceb589dcc52956b16a6f0c259aebdad
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>