Fix valid warnings given at warning level 4 or add #pragma to silence them

Fix valid warnings given at warning level 4 or add #pragma to silence them. When building with MSVC 2005 and -W4 there are some warnings that appear which are actually silenced in other places or may be valid ones. For example qstringbuilder.h warns about not being able to provide an assignment operator by default for the class.

There are also cases where the "conditional expression is constant " being warned, so a general compile with -W4 so the valid ones can be resolved would be a good idea.