Notify attached UIs whenever menus change

This adds a redraw notification "update_menu" which is sent whenever
Vim's menus are changed by the :menu command and friends.

test/functional/api/menu_spec.lua

@@ -0,0 +1,52 @@
+local helpers = require('test.functional.helpers')
+local Screen = require('test.functional.ui.screen')
+
+local clear = helpers.clear
+local command = helpers.command
+local feed = helpers.feed
+
+describe("update_menu notification", function()
+
+  local screen
+
+  before_each(function()
+    clear()
+    screen = Screen.new()
+    screen:attach()
+  end)
+
+  after_each(function()
+    screen:detach()
+  end)
+
+  function expect_sent(expected)
+    screen:wait(function()
+      if screen.update_menu ~= expected then
+        if expected then
+          return 'update_menu was expected but not sent'
+        else
+          return 'update_menu was sent unexpectedly'
+        end
+      end
+    end)
+  end
+
+  it("should be sent when adding a menu", function()
+    command('menu Test.Test :')
+    expect_sent(true)
+  end)
+
+  it("should be sent when deleting a menu", function()
+    command('menu Test.Test :')
+    screen.update_menu = false
+
+    command('unmenu Test.Test')
+    expect_sent(true)
+  end)
+
+  it("should not be sent unnecessarily", function()
+    feed('i12345<ESC>:redraw<CR>')
+    expect_sent(false)
+  end)
+
+end)