Display wide characters correctly.

- Made termstr's length match the on-screen width.
  - Achieved by adding a zero width character after each wide
    character.
  - This fixes ljust and rjust, so that there is correct padding,
    otherwise Portal appearances aren't the correct width.
- When getting a sub-string containing half a wide character,
  the half character is a space.
- Some wide characters are still broken. e.g. country flags

tests/termstr_test.py

@@ -101,6 +101,11 @@ class TermStrTests(unittest.TestCase):
         self.assertEqual(foo_bold.ljust(5), foo_bold + TermStr("  "))
         self.assertEqual(foo_bold.rjust(0), foo_bold)
         self.assertEqual(foo_bold.rjust(5), TermStr("  ") + foo_bold)
+        baz = TermStr("b👋z")
+        self.assertEqual(len(baz), 4)
+        self.assertEqual(baz[3:], TermStr("z"))
+        self.assertEqual(baz[:2], TermStr("b "))
+        self.assertEqual(baz[2:], TermStr(" z"))
 
     def test_from_term(self):
         def test_round_trip(term_str):