Revise FileI and SoundStreamT interfaces to use more appropriate datatypes

[Carsten]

The current interfaces (especially Read() and related methods) use datatypes that require type casts in several places of user code. The use of type size_t (or similar) might be more appropriate for these method signatures.

The implementation of this ticket should make sure that all occurrences of redundant typecasts are removed; e.g. temporarily change the signature of all revised methods to find all places of use.

Examples:

// Note that the new signature causes the compiler to point out all places of use,
// so that now redundant type-casts can be spotted and cleaned-up.
size_t SoundStreamT::Read(bool& Success, unsigned char* Buffer, size_t Size)=0;

// As above, but temp is removed again when all user code has been identified and updated.
void OutFileI::Write(const char* Buffer, size_t Size, std::string temp)=0;