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/api/vim.c

@@ -57,6 +57,7 @@
 #include "nvim/popupmenu.h"
 #include "nvim/runtime.h"
 #include "nvim/state.h"
+#include "nvim/statusline.h"
 #include "nvim/types.h"
 #include "nvim/ui.h"
 #include "nvim/vim.h"