Blender Git Loki

Data transfer: make operator able to work 'reversed' (i.e. transfer from selected objects to active).

Needed to replace weight transfer modifier in WeightPaint mode...

Note this is not exposed to users in UI, shall remain technical intern
parameter imho. Esp. since behavior when several sources is a bit 'random'
(merely uses each source in selection order...).

Also, this correct a bug, where 'lib' linked objects/meshes could not be used
as source...