comctl32: toolbar: Make sure all TBUTTON_INFO fields are initialized when a button is added in the middle.

The dde.c tests succeed on Windows and the DDE callback registered is
set to trigger on all messages it gets. Thus it looks like Windows
doesn't calls the callback in that case.

The code for the child windows works now for the standalone windows as well.

Also handle the case where TMP and TEMP are not defined.