Text: Experimental bug fix (related to font_ management)

* Initialize position & size of base class to Point2i::null,
Size2i::null (respectively). This was the original intention.

* Use set_font method for the font_ resource var managing when
the copy constructor or copy assignment operator is called. This
might help potential crashes that could occur -- class var
font_ could end up being NULL if we do not perform a 'deep copy',
as per what we noticed while debugging related issues in nom::Window.
Introduce stub files for class object DataViewBitmapColumn