brasero-src-selection: wire in constructed handler

Fixes 33520f866236b54e9c87650a1ff6fa84c98a616c
brasero-tool-dialog: don't populate the model until after construction

As has been done for other BraseroMediumSelection subclasses, works around
a drawing issue.
README: minor tweaks, we use GStreamer 1.0 now
NEWS: update for a 3.10 release
Bump version to 3.10