INSERTITEM should fail on a zero mask. If in GETITEM the mask has an
unknown field it should store only the comctl32 4.0 fields (with test
case).

- '%hs' in widechar string functions is wrongly recognized as
  a widechar string. Fix this by always checking the prefix first.
- add testcase
- remove code duplication in %c/%s handling by moving code into a function.

Move ShowWindow tests for children with an invisible parent into a
separate test, add more ShowWindow tests. The tests show that an
assumption in dlls/x11drv/winpos.c that SetWindowPos should not be
called on a child with an invisible parent is not always true.

common ComputeConformance function to store the result in
pStubMsg->MaxCount (as this is what callback function store the
conformance value into anyway).

The conformance has to be the first thing in the buffer, in accordance 
with the DCE/RPC spec and to be compatible with the pointer layout 
format strings that MIDL generates.

array functions, before calling a function that could change it and
cause the wrong number of loop iterations.