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.
