[text] Rename :alignment to :line-alignment

[text] Rename :alignment to :line-alignment

The :alignment property is prone to generate confusion: developers
will set it thinking that the contents of a ClutterText will
automagically align themselves.

Instead of using the generic term :alignment, and following the
GTK+ convention, we should use a more specific term, conveying the
actual effect of the property: alignment of the lines with respect
to each other, and not to the overall allocated area.

See bug 1428:

  http://bugzilla.openedhand.com/show_bug.cgi?id=1428