Optimize editing large files

- Fixed slow editing on large files.
  - Only calculating appearance of lines that are seen.
  - Not resizing all lines when max line length changes.
- Fast changing of syntax highlighting themes.
- Faster startup, since not highlighting all lines.

tests/diff_editor_test.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+
+
+import unittest
+
+import diff_edit
+
+
+class OverlayListTestCase(unittest.TestCase):
+
+    def test_overlay_list(self):
+        self.assertEqual(diff_edit.overlay_list([1, 2, 3, 4], [5, 6], 0), [5, 6, 3, 4])
+        self.assertEqual(diff_edit.overlay_list([1, 2, 3, 4], [5, 6], 3), [1, 2, 3, 5])
+        self.assertEqual(diff_edit.overlay_list([1, 2, 3, 4], [5, 6], -1), [6, 2, 3, 4])
+        self.assertEqual(diff_edit.overlay_list([5, 6], [1, 2, 3, 4], -1), [2, 3])
+
+    
+if __name__ == "__main__":
+    unittest.main()