Implemented reporting callbacks.

Both the dependency Tracer class and the Packer class now support a
callback object, where the latter is a subclass of the former.

For file transfers running in a separate thread, there is a thread-safe
wrapper for progress callbacks. This wrapper can be called from any thread,
and calls the wrapped callback object from the main thread. This way the
callback implementation itself doesn't have to worry about threading
issues.

tests/test_pack_progress.py

@@ -0,0 +1,42 @@
+import threading
+import time
+import typing
+import unittest
+from pathlib import Path
+from unittest import mock
+
+from blender_asset_tracer.pack import progress
+
+
+class ThreadedProgressTest(unittest.TestCase):
+    def test_threaded_progress(self):
+        cb = mock.Mock(progress.Callback)
+        tscb = progress.ThreadSafeCallback(typing.cast(progress.Callback, cb))
+
+        # Flushing an empty queue should be fast.
+        before = time.time()
+        tscb.flush(timeout=1)
+        duration = time.time() - before
+        self.assertLess(duration, 1)
+
+        def thread():
+            tscb.pack_start()
+            tscb.pack_done(Path('one'), {Path('two'), Path('three')})
+            tscb.trace_asset(Path('four'))
+            tscb.transfer_file(Path('five'), Path('six'))
+            tscb.transfer_file_skipped(Path('seven'), Path('eight'))
+            tscb.transfer_progress(327, 47)
+            tscb.missing_file(Path('nine'))
+
+        t = threading.Thread(target=thread)
+        t.start()
+        t.join(timeout=3)
+        tscb.flush(timeout=3)
+
+        cb.pack_start.assert_called_with()
+        cb.pack_done.assert_called_with(Path('one'), {Path('two'), Path('three')})
+        cb.trace_asset.assert_called_with(Path('four'))
+        cb.transfer_file.assert_called_with(Path('five'), Path('six'))
+        cb.transfer_file_skipped.assert_called_with(Path('seven'), Path('eight'))
+        cb.transfer_progress.assert_called_with(327, 47)
+        cb.missing_file.assert_called_with(Path('nine'))