Sets the font and font size to be used for text.
This function works by changing the DefaultAppearance for
the field using the specified FontObject and size.
Properties such ase the TextFont and TextSize get the effective value
which may be inherited. Calling this funtion sets the value on this
item itself which will override any inherited value.
Passing a font value of null will remove both the font and size
from the item itself though a value may be still be present if it
is inherited from a parent field or from document defaults.
A zero size font indicates that the font should scale so that it
fits the size of the field. Negative font heights appear the same
as positive ones but are best avoided as they are likely to cause
The appearance of the field is not updated until you change the
Value or call UpdateAppearance.