This was already supported in "Select" & "Deselect" but not in "Assign".
So similar to rB6b39dc7672eb, we now check if the material corresponding
to the currently selected material slot is found in other objects
materials and assign this (instead of always assigning their 'own'
actcol).

sidenote: couldnt we use BKE_object_material_slot_find_index in both
material_slot_assign_exec & material_slot_de_select?