EditFont's use of Curve.len was very confusing, in editmode it
represented the number of characters, in object mode the number of
bytes. add Curve.len_wchar and keep track of both.
Also don't convert the editmode text into utf8 on every keystroke.
Now this is done on exiting editmode or save - to match most other
object types.
This also fixes curves 'body_format' being reported with an invalid size.
15bc30f4ee