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/misc1.c

@@ -2714,7 +2714,7 @@ int call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg)
   if (p_verbose > 3) {
     verbose_enter();
     smsg(_("Calling shell to execute: \"%s\""), cmd == NULL ? p_sh : cmd);
-    ui_linefeed();
+    msg_putchar('\n');
     verbose_leave();
   }