Inscription: Derive row alignment from xalign

Inscription: Derive row alignment from xalign

Texts usually want the alignment of each row to match the xalign of
the text itself.

Derive the alignment of the PangoLayout from the xalign property of
the inscription. Because Pango doesn't provide float row alignment,
map left, center and right from the xalign in 1 / 3 steps.