mantidpython.bat uses Qt4 by default

Matches the behaviour of the linux scripts

buildconfig/CMake/Packaging/mantidpython.bat.in

@@ -21,6 +21,11 @@ set PYTHONPATH=%MANTIDPATH%;%PYTHONPATH%@PARAVIEW_PYTHON_PATHS@
 set _PYTHON_EXE=%PYTHONHOME%\python.exe
 set _IPYTHON_CMD=%PYTHONHOME%\Scripts\ipython.cmd
 
+:: QtPy should use Qt4 by unless specified
+if "%QT_API%"=="" (
+  set QT_API=pyqt
+)
+
 :: Matplotlib backend should default to Qt if not set (requires matplotlib >= 1.5)
 if "%MPLBACKEND%"=="" (
   set MPLBACKEND=qt4agg