Sort out deprecation of qVariant{From,Set}Value()

Fom development@ discussion, first chehrlic:

[...] the two functions qVariantFromValue() and qVariantSetValue() are deprecated but the replacements QVariant::setValue() / fromValue() are using exactly those two functions...
Those two functions are some of the last obsolete functions which did not yet get decorated with QT_DEPRECATED but since this is affects a major class of Qt I don't think it's a task for me.

to which peppe replied:

Could it simply be possible to move the implementation of qVariantSetValue to QVariant::setValue, and then have the former call the latter, instead of viceversa?

That sounds like a plausible solution; we clearly need to look into this.