refactor: move statusline code from buffer.c and [draw]screen.c to new file

problem: code for drawing statusline is arbitrarily spreadout between drawscreen.c, screen.c and buffer.c
solution: move it to a new file statusline.c

- rename archaic internal name "status match" to public name "wildmenu"
- showruler() does not show the ruler. it show anything which displays
  info about the cursor. Rename it accordingy.

src/nvim/buffer.h

@@ -62,6 +62,9 @@ enum bfa_values {
   BFA_IGNORE_ABORT = 8,  // do not abort for aborting()
 };
 
+EXTERN char *msg_loclist INIT(= N_("[Location List]"));
+EXTERN char *msg_qflist INIT(= N_("[Quickfix List]"));
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "buffer.h.generated.h"
 #endif