Made ConvertToScreen() methods const.
Added a Window::ConvertFromScreen() method and used instead of
duplicating code.
Add sint32
in Bitmap::FillPolygon(): clamp the y coordinate to 0 in advance, and
check also in advance if the x coordinate is < 0, before adding to the
node list.
Correct IE::text_area type.
Removed TODO
Implement TextArea::SetText
Used a TextArea for writing some text (for test)