A TextBox is used for drawing text, either
inside a rectangular box or by flowing it around objects of a Frame.
This class uses TextLayouter behind
the scenes to do the hard work.
Creates a new TextBox object with the given
inline items (e.g. TextFragment and InlineBox objects).
Fits the text box into the Frame.
Depending on the 'position' style property, the text is either fit
into the rectangular area given by available_width and
available_height, or fit to the outline of the frame starting
from the top.