brasero-src-selection: don't populate the model until after construction

As has already been done for brasero-drive-selection and
brasero-dest-selection.
brasero-jacket-edit: generalise text justification to reduce duplication
brasero-misc: tweak BRASERO_UTILS_LOG to behave more conformantly

BRASERO_UTILS_LOG should behave similarly to BRASERO_BURN_LOG and
BRASERO_MEDIA_LOG, writing to stdout using printf rather than using glogv
with a utils specific log domain.

In future all three macros should use GLib's logging functionality
brasero-misc: expose a GOption for enabling brasero utils debugging
Add brasero-utils GOptionGroup to main context

This exposes runtime enabling of logging from libbrasero-utils
brasero-medium-properties: add Cancel button to image properties dialogue

Otherwise the user can't close the dialogue without changing the image
properties.