wininet: Rename WININET_strdup_AtoW to heap_strdupAtoW and handle NULL argument in its implementation.

This is for SM4 constant buffers, which look like cb<x>[<y>]. At some later
point we should probably translate pre-SM4 constants to constant buffers.

The code removed by this patch was a leftover from the time when we tried to
emulate MOVA with a plain ARL, which only supports one source in plain ARB.
This breaks the more sophisticated MOVA and 4 component ADDR register we have
now however.

comctl32: Explicitly initialize visible order of a newly added item, force visible order recalculation on redraw.