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.
Now the time for importing the kitty.config module has been halved, from
16ms from 32ms on my machine. Also, the new architecture will eventually
allow for auto generating a bunch of python-to-C boilerplate code.
