Add automatic index rebuild (#975)

* Rebuild the index automatically if not all files are in the index on open
* Update tests

tests/test_core/test_core_index.py

@@ -108,7 +108,7 @@ def testCoreIndex_LoadSave(monkeypatch, nwLipsum, mockGUI, outDir, refDir):
     # Break the index and check that we notice
     assert theIndex.indexBroken is False
     theIndex._tagIndex["Bod"].append("Stuff")
-    theIndex.checkIndex()
+    theIndex._checkIndex()
     assert theIndex.indexBroken is True
 
     # Finalise