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/fe-text/meson.build

@@ -23,6 +23,7 @@ executable('irssi',
     'statusbar.c',
     'term.c',
     'textbuffer-commands.c',
+    'textbuffer-formats.c',
     'textbuffer-view.c',
     'textbuffer.c',
   )
@@ -53,6 +54,7 @@ install_headers(
     'statusbar-item.h',
     'statusbar.h',
     'term.h',
+    'textbuffer-formats.h',
     'textbuffer-view.h',
     'textbuffer.h',
   ),