change FilesDB to singleton class

move hash calculation back in to Files class
clear cache now clears hash cache in addition to picture cache

core/app.py

@@ -139,7 +139,8 @@ class DupeGuru(Broadcaster):
         self.discarded_file_count = 0
         self.exclude_list = ExcludeList()
         hash_cache_file = op.join(self.appdata, "hash_cache.db")
-        self.directories = directories.Directories(self.exclude_list, hash_cache_file)
+        fs.filesdb.connect(hash_cache_file)
+        self.directories = directories.Directories(self.exclude_list)
         self.results = results.Results(self)
         self.ignore_list = IgnoreList()
         # In addition to "app-level" options, this dictionary also holds options that will be
@@ -422,6 +423,9 @@ class DupeGuru(Broadcaster):
         except FileNotFoundError:
             pass  # we don't care
 
+    def clear_hash_cache(self):
+        fs.filesdb.clear()
+
     def copy_or_move(self, dupe, copy: bool, destination: str, dest_type: DestType):
         source_path = dupe.path
         location_path = first(p for p in self.directories if dupe.path in p)
@@ -753,6 +757,9 @@ class DupeGuru(Broadcaster):
         self.exclude_list.save_to_xml(p)
         self.notify("save_session")
 
+    def close(self):
+        fs.filesdb.close()
+
     def save_as(self, filename):
         """Save results in ``filename``.