Add ascent, descent and leading to TextLayout.

Untested, but the values were already there, they're just being exposed. Also, Flash and HTML are
unimplemented.
Add method to get the bounds of a single line of layed out text.

This is needed to support underlining in TriplePlay. Unimplemented in HTML and Flash.
Use ascent and descent instead of relying on getBounds()
Account for pad in our line bounds