Label needs horizontal alignment

[ecton]

(Related to #128)

Currently, label draws its text using TextOrigin::Center. This isn't really correct, as it implies that we're centering the text. However, it allows a reasonable default behavior, and works well with align_left/align_right. But, if the text wraps, the wrapped lines are not actually aligned respective to the containing element. They are left-aligned.

The real solution is to support horizontal alignment and have Label honor the current setting. This will likely have changes that cascade into Kludgine since that's where text layout occurs and it doesn't support these alignment options either.