Previously the `callback.trace_blendfile()` callback was called just before
tracing its dependencies would start, i.e. after opening it. This is now
changed to before opening it, because that can take a long time (to load
SDNA). this will make any UI (like the Flamenco add-on) report the right
filename when waiting for big files, instead of lingering on the
last-opened (potentially very small) blend file.
cc4043969e