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.

gen-wcwidth.py

@@ -529,6 +529,52 @@ def gen_wcwidth() -> None:
     subprocess.check_call(['gofmt', '-w', '-s', gof.name])
 
 
+def gen_rowcolumn_diacritics() -> None:
+    # codes of all row/column diacritics
+    codes = []
+    with open("./rowcolumn-diacritics.txt", "r") as file:
+        for line in file.readlines():
+            if line.startswith('#'):
+                continue
+            code = int(line.split(";")[0], 16)
+            codes.append(code)
+
+    with create_header('kitty/rowcolumn-diacritics.c') as p:
+        p('#include "unicode-data.h"')
+        p('int diacritic_to_num(char_type code) {')
+        p('\tswitch (code) {')
+
+        range_start_num = 1
+        range_start = 0
+        range_end = 0
+
+        def print_range() -> None:
+            if range_start >= range_end:
+                return
+            write_case((range_start, range_end), p)
+            p('\t\treturn code - ' + hex(range_start) + ' + ' +
+              str(range_start_num) + ';')
+
+        for code in codes:
+            if range_end == code:
+                range_end += 1
+            else:
+                print_range()
+                range_start_num += range_end - range_start
+                range_start = code
+                range_end = code + 1
+        print_range()
+
+        p('\t}')
+        p('\treturn 0;')
+        p('}')
+
+    with open('kittens/icat/rowcolumn_diacritics.py', 'w') as f:
+        f.write('# generated by gen-wcwidth.py, do not edit\n\n')
+        strings_utf8 = ', '.join(repr(chr(c).encode('utf-8')) for c in codes)
+        f.write("rowcolumn_diacritics_utf8 = ({})  # noqa\n".format(strings_utf8))
+
+
 parse_ucd()
 parse_prop_list()
 parse_emoji()
@@ -537,3 +583,4 @@ gen_ucd()
 gen_wcwidth()
 gen_emoji()
 gen_names()
+gen_rowcolumn_diacritics()