refactor(screen): screen.c delenda est

drawscreen.c vs screen.c makes absolutely no sense.
The screen exists only to draw upon it, therefore helper functions
are distributed randomly between screen.c and the file that
does the redrawing. In addition screen.c does a lot of drawing on the
screen.

It made more sense for vim/vim as our grid.c is their screen.c

Not sure if we want to dump all the code for option chars into
optionstr.c, so keep these in a optionchar.c for now.

src/nvim/digraph.c

@@ -15,6 +15,7 @@
 #include "nvim/charset.h"
 #include "nvim/digraph.h"
 #include "nvim/drawscreen.h"
+#include "nvim/eval.h"
 #include "nvim/eval/typval.h"
 #include "nvim/eval/typval_defs.h"
 #include "nvim/ex_cmds_defs.h"
@@ -2180,3 +2181,42 @@ static void keymap_unload(void)
   curbuf->b_kmap_state &= ~KEYMAP_LOADED;
   status_redraw_curbuf();
 }
+
+/// Get the value to show for the language mappings, active 'keymap'.
+///
+/// @param fmt  format string containing one %s item
+/// @param buf  buffer for the result
+/// @param len  length of buffer
+bool get_keymap_str(win_T *wp, char *fmt, char *buf, int len)
+{
+  char *p;
+
+  if (wp->w_buffer->b_p_iminsert != B_IMODE_LMAP) {
+    return false;
+  }
+
+  buf_T *old_curbuf = curbuf;
+  win_T *old_curwin = curwin;
+  char *s;
+
+  curbuf = wp->w_buffer;
+  curwin = wp;
+  STRCPY(buf, "b:keymap_name");       // must be writable
+  emsg_skip++;
+  s = p = eval_to_string(buf, NULL, false);
+  emsg_skip--;
+  curbuf = old_curbuf;
+  curwin = old_curwin;
+  if (p == NULL || *p == NUL) {
+    if (wp->w_buffer->b_kmap_state & KEYMAP_LOADED) {
+      p = wp->w_buffer->b_p_keymap;
+    } else {
+      p = "lang";
+    }
+  }
+  if (vim_snprintf(buf, (size_t)len, fmt, p) > len - 1) {
+    buf[0] = NUL;
+  }
+  xfree(s);
+  return buf[0] != NUL;
+}