Add text justification

Add text justification

Subtitle recommendations often include that multi line
subtitles should be left justified as this is easier for
the eyes. This is also the standard used by several television
companies.

This add the possibility to define how subtitles are to
be justified, independently of where they are aligned.
The most common way could be to set justify to left, and have
alignment to center. But you can, for example, have alignment
to left and justify to center, giving subtitles to the left but
justifed on the center (instead of normal left justified).
Using justify right and alignment of center, might be good
choice for Arabic.

If justify is not defined, all works like before.
If justify is defined, subtitles are aligned as defined
by alignment and justified as defined by justify.

ASS is not extended by this, justify can only be defined
by setting Justify to wanted justification.