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/textbuffer-view.h

@@ -27,6 +27,7 @@ typedef struct {
 typedef struct {
 	time_t last_access;
 
+	char *line_text;
 	int count; /* number of real lines */
 
 	/* variable sized array, actually. starts from the second line,