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/vim_spec.lua

@@ -1242,6 +1242,8 @@ describe('API', function()
           ext_popupmenu = false,
           ext_tabline = false,
           ext_wildmenu = false,
+          ext_newgrid = screen._options.ext_newgrid or false,
+          ext_hlstate=false,
           height = 4,
           rgb = true,
           width = 20,
@@ -1252,18 +1254,9 @@ describe('API', function()
       screen:detach()
       screen = Screen.new(44, 99)
       screen:attach({ rgb = false })
-      expected = {
-        {
-          chan = 1,
-          ext_cmdline = false,
-          ext_popupmenu = false,
-          ext_tabline = false,
-          ext_wildmenu = false,
-          height = 99,
-          rgb = false,
-          width = 44,
-        }
-      }
+      expected[1].rgb = false
+      expected[1].width = 44
+      expected[1].height = 99
       eq(expected, nvim("list_uis"))
     end)
   end)