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.

test/functional/api/version_spec.lua

@@ -156,6 +156,6 @@ describe("ui_options in metadata", function()
     local api = helpers.call('api_info')
     local options = api.ui_options
     eq({'rgb', 'ext_cmdline', 'ext_popupmenu',
-        'ext_tabline', 'ext_wildmenu'}, options)
+        'ext_tabline', 'ext_wildmenu', 'ext_newgrid', 'ext_hlstate'}, options)
   end)
 end)