New RenderAligningShiftedBox abstract class for render boxes with
"alignment", which as of now is:
- RenderPositionedBox
- RenderConstrainedOverflowBox (ex RenderOverflowBox)
- RenderSizedOverflowBox
- RenderFractionallySizedOverflowBox (ex RenderFractionallySizedBox)
(The respective widgets are Align, OverflowBox, SizedOverflowBox, and
FractionallySizedBox. They haven't been renamed.)
Also I added some toString details.
Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.
This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.
