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/ex_docmd.c

@@ -6320,8 +6320,10 @@ static void ex_stop(exarg_T *eap)
       autowrite_all();
     }
     apply_autocmds(EVENT_VIMSUSPEND, NULL, NULL, false, NULL);
+
+    // TODO(bfredl): the TUI should do this on suspend
     ui_cursor_goto((int)Rows - 1, 0);
-    ui_linefeed();
+    ui_call_grid_scroll(1, 0, Rows, 0, Columns, 1, 0);
     ui_flush();
     ui_call_suspend();  // call machine specific function