make lines reformattable

- completely removed the old textbuffer representation (
  https://github.com/shabble/irssi-docs/wiki/Notes-256-Colour#textbuffer-encoding
  )

- textbuffer-formats is an extra module, so if we unhook the signals it
  should go back to the "old way" of storing pre-rendered tex

- design uses cache, original formats and list of arguments

src/core/levels.h

@@ -6,6 +6,7 @@
    difficult message leveling system (which might be done if really
    needed..). */
 
+/* clang-format off */
 /* Message levels */
 enum {
 	MSGLEVEL_CRAP         = 0x0000001,
@@ -38,8 +39,12 @@ enum {
 	MSGLEVEL_NEVER        = 0x4000000, /* never ignore / never log */
 	MSGLEVEL_LASTLOG      = 0x8000000, /* used for /lastlog */
 
-	MSGLEVEL_HIDDEN       = 0x10000000 /* Hidden from view */
+	MSGLEVEL_HIDDEN       = 0x10000000, /* Hidden from view */
+	MSGLEVEL_RESERVED1    = 0x20000000,
+	MSGLEVEL_RESERVED2    = 0x40000000,
+	MSGLEVEL_FORMAT       = 0x80000000  /* Format data */
 };
+/* clang-format on */
 
 int level_get(const char *level);
 int level2bits(const char *level, int *errorp);