ui: use line-based rather than char-based updates in screen.c

Add ext_newgrid and ext_hlstate extensions. These use predefined
highlights and line-segment based updates, for efficiency and
simplicity.. The ext_hlstate extension in addition allows semantic
identification of builtin and syntax highlights.

Reimplement the old char-based updates in the remote UI layer, for
compatibility. For the moment, this is still the default. The bulitin
TUI uses the new line-based protocol.

cmdline uses curwin cursor position when ext_cmdline is active.

src/nvim/popupmnu.c

@@ -341,6 +341,8 @@ void pum_redraw(void)
     idx = i + pum_first;
     attr = (idx == pum_selected) ? attr_select : attr_norm;
 
+    screen_puts_line_start(row);
+
     // prepend a space if there is room
     if (curwin->w_p_rl) {
       if (pum_col < curwin->w_wincol + curwin->w_width - 1) {
@@ -488,6 +490,7 @@ void pum_redraw(void)
                        ? attr_thumb : attr_scroll);
       }
     }
+    screen_puts_line_flush(false);
     row++;
   }
 }