[QTBUG-13230] Make QVariantAnimation non-abstract

[QTBUG-13230] Make QVariantAnimation non-abstract

The bug report argues convincingly that, but for the pure virtual
updateCurrentValue(QVariant) function, QVariantAnimation is quite
usable as-is.

So add an empty implementation.

Reported-by: André Somers <andre@familiesomers.nl>
Task-number: QTBUG-13230
Change-Id: I689b4a80fc7d5dfae67c2255777f89afb16f9c21
Reviewed-by: Andre Somers <andre@familiesomers.nl>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>