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/search.c

@@ -2423,7 +2423,7 @@ void showmatch(int c)
       curwin->w_cursor = mpos;          // move to matching char
       *so = 0;                          // don't use 'scrolloff' here
       *siso = 0;                        // don't use 'sidescrolloff' here
-      showruler(false);
+      show_cursor_info(false);
       setcursor();
       ui_flush();
       // Restore dollar_vcol(), because setcursor() may call curs_rows()