Move to project directory before using xgettext

This is done in order to hide the absolute path in regenerated
.po and .pot files.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Include TRANS comments for activities

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Do not update if no stores detected

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>