Image placement using Unicode placeholders

This commit introduces the Unicode placeholder image placement method.
In particular:
- Virtual placements can be created by passing `U=1` in a put command.
- Images with virtual placements can be displayed using the placeholder
  character `U+10EEEE` with diacritics indicating rows and columns.
- The image ID is indicated by the foreground color of the placeholder.
  Additionally, the most significant byte of the ID can be specified via
  the third diacritic.
- Underline color can be optionally used to specify the placement ID.
- A bug was fixed, which caused incomplete image removal when it was
  overwritten by another image with the same ID.

kitty_tests/parser.py

@@ -410,7 +410,7 @@ class TestParser(BaseTest):
             for f in 'action delete_action transmission_type compressed'.split():
                 k.setdefault(f, b'\0')
             for f in ('format more id data_sz data_offset width height x_offset y_offset data_height data_width cursor_movement'
-                      ' num_cells num_lines cell_x_offset cell_y_offset z_index placement_id image_number quiet').split():
+                      ' num_cells num_lines cell_x_offset cell_y_offset z_index placement_id image_number quiet unicode_placement').split():
                 k.setdefault(f, 0)
             p = k.pop('payload', '').encode('utf-8')
             k['payload_sz'] = len(p)