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/unicode-data.h

@@ -5,6 +5,9 @@
 static const combining_type VS15 = 1364, VS16 = 1365;
 // END_KNOWN_MARKS
 
+// Converts row/column diacritics to numbers.
+int diacritic_to_num(char_type ch);
+
 bool is_combining_char(char_type ch);
 bool is_ignored_char(char_type ch);
 bool is_word_char(char_type ch);