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/ui/options_spec.lua

@@ -30,10 +30,15 @@ describe('ui receives option updates', function()
     ext_popupmenu=false,
     ext_tabline=false,
     ext_wildmenu=false,
+    ext_newgrid=false,
+    ext_hlstate=false,
   }
 
   it("for defaults", function()
     screen:attach()
+    -- NB: UI test suite can be run in both "newgrid" and legacy grid mode.
+    -- In both cases check that the received value is the one requested.
+    defaults.ext_newgrid = screen._options.ext_newgrid or false
     screen:expect(function()
       eq(defaults, screen.options)
     end)
@@ -41,6 +46,7 @@ describe('ui receives option updates', function()
 
   it("when setting options", function()
     screen:attach()
+    defaults.ext_newgrid = screen._options.ext_newgrid or false
     local changed = {}
     for k,v in pairs(defaults) do
       changed[k] = v
@@ -89,6 +95,7 @@ describe('ui receives option updates', function()
     end
 
     screen:attach({ext_cmdline=true, ext_wildmenu=true})
+    defaults.ext_newgrid = screen._options.ext_newgrid or false
     changed.ext_cmdline = true
     changed.ext_wildmenu = true
     screen:expect(function()