Fix SIGSEGV handling paint event in wxQT

The logic was checking for m_qtPicture but used m_qtPaintBuffer (that is used bellow in a alternate block)
Although the segmentation fail is fixed, this should be revised as it seems to be an explicit QtPaintClientDCPicture with this logic
Make calendar control helper internal / private

 * Declaration of wxQtCalendarWidget moved from header to c++ source file
 * Renamed method to match signal names
Fix qt to wx date conversion function
Make spin control helper internal / private in wxQT

 * Move helper (wxQtSpinBoxBase et al) declaration to c++ sources
 * Use QSpinBox / QDoubleSpinBox for header declarations (instead of the helper)
 * Refactor Create to instantiate the helper in the derived classes of wxSpinCtrlBase
 * Use a dynamic_cast in SetValue(wxString&) to safely access the derived class helper
 * Make SetValue (overloaded) virtual and reimplement both to avoid hiding
 * Change the controls sample to test SetValue(wxString&)

Note that the dynamic_cast is needed now to access QSpinBox::valueFromText that is protected and is redeclared in the helper as public (all the other heplers didn't have this requeriment so far).