Fix build on Windows

* Include stdint.h for uint16_t and friends
* Fix struct vs. class mismatch
* Export QV4::String as referenced by QQuick
* Eliminate declared but not defined destructor
* Fix v4classgen python script to not require argsparse, so that it works with older Python versions
* Replace the use of QStringLiteral in some places where we concatenate the
  input (like "Foo" \n "Bar") and MSVC thinks that the two string literals have
  a different "width" (8 bit vs. 16 bit) and therefore cannot be used with QStringLiteral
* Add src/qquick to the include search path to work around the differing include search path
  semantics between MSVC and gcc

Change-Id: Iedcc21cc757c53b6c7d81b514fd857ff1f573b01
Reviewed-by: Lars Knoll <lars.knoll@digia.com>