Implement missing font info constructor in wxQT

(This is also needed by the drawing sample)
Implement missing font family, weight & style getters
Fix font weight and underline setters
Fix font face name getter (using actual info from Qt)

It was returning an empty string (if no facename was given at the font ctor), and that caused a segmentation fault in cairo_show_text
Fix wxQtDCImpl::GetHandle() to return QPainter

This is needed by the grahpic context
Fix cairo image buffer paint DC in wxQT

* do not destroy the image as it holds the pixel data buffer
* hence, do not create a new image as it isn't necessary

This solution is similar to cairo qt surface internals, that stores image and image_equiv -the cairo image surface-.