Add .show attr to Button; False when dropdown is empty

Fixes #3138

wagtail/wagtailadmin/templates/wagtailadmin/pages/listing/_buttons.html

@@ -1,3 +1,5 @@
 {% for button in buttons %}
+    {% if button.show %}
     <li>{{ button|safe }}</li>
+    {% endif %}
 {% endfor %}

wagtail/wagtailadmin/widgets.py

@@ -10,6 +10,7 @@ from django.forms.utils import flatatt
 from django.template.loader import render_to_string
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.formats import get_format
+from django.utils.functional import cached_property
 from django.utils.html import format_html
 from django.utils.translation import ugettext_lazy as _
 from taggit.forms import TagWidget
@@ -191,6 +192,8 @@ class AdminPageChooser(AdminChooser):
 @python_2_unicode_compatible
 @total_ordering
 class Button(object):
+    show = True
+
     def __init__(self, label, url, classes=set(), attrs={}, priority=1000):
         self.label = label
         self.url = url
@@ -234,12 +237,13 @@ class BaseDropdownMenuButton(Button):
     def __init__(self, *args, **kwargs):
         super(BaseDropdownMenuButton, self).__init__(*args, url=None, **kwargs)
 
-    def get_buttons_in_dropdown(self):
+    @cached_property
+    def dropdown_buttons(self):
         raise NotImplementedError
 
     def render(self):
         return render_to_string(self.template_name, {
-            'buttons': self.get_buttons_in_dropdown(),
+            'buttons': self.dropdown_buttons,
             'label': self.label,
             'title': self.attrs.get('title'),
             'is_parent': self.is_parent})
@@ -256,7 +260,12 @@ class ButtonWithDropdownFromHook(BaseDropdownMenuButton):
 
         super(ButtonWithDropdownFromHook, self).__init__(label, **kwargs)
 
-    def get_buttons_in_dropdown(self):
+    @property
+    def show(self):
+        return bool(self.dropdown_buttons)
+
+    @cached_property
+    def dropdown_buttons(self):
         button_hooks = hooks.get_hooks(self.hook_name)
         return sorted(itertools.chain.from_iterable(
             hook(self.page, self.page_perms, self.is_parent)