Replace appmenu with window-local menu

There's an initiave in GNOME to replace app menus with "in-window"
menus: https://gitlab.gnome.org/GNOME/Initiatives/issues/4

This commit brings Piper in line with this new approach.

Fixes #267.

data/piper.gresource.xml

@@ -18,7 +18,5 @@
         <file preprocess="xml-stripblanks">ui/ResolutionsPage.ui</file>
         <file preprocess="xml-stripblanks">ui/WelcomePerspective.ui</file>
         <file preprocess="xml-stripblanks">ui/Window.ui</file>
-        <!-- Using this alias, GtkApplication will automatically pick it up for us. -->
-        <file alias="gtk/menus.ui" preprocess="xml-stripblanks">ui/Menus.ui</file>
     </gresource>
 </gresources>

data/ui/Menus.ui

@@ -1,17 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<interface>
-  <menu id="app-menu">
-    <section>
-      <item>
-        <attribute name="action">app.about</attribute>
-        <attribute name="label" translatable="yes">_About</attribute>
-      </item>
-      <item>
-        <attribute name="action">app.quit</attribute>
-        <attribute name="label" translatable="yes">_Quit</attribute>
-        <attribute name="accel">&lt;Primary&gt;q</attribute>
-      </item>
-    </section>
-  </menu>
-</interface>
-

data/ui/Window.ui

@@ -2,6 +2,12 @@
 <!-- Generated with glade 3.20.0 -->
 <interface>
   <requires lib="gtk+" version="3.20"/>
+  <menu id="primary_menu">
+    <item>
+      <attribute name="action">app.about</attribute>
+      <attribute name="label" translatable="yes">_About Piper</attribute>
+    </item>
+  </menu>
   <template class="Window" parent="GtkApplicationWindow">
     <!-- This size is determined to be the best one with the current set of
 	 device SVGs in libratbag. It is set to ensure that the jump in size

piper/window.py

@@ -36,6 +36,7 @@ class Window(Gtk.ApplicationWindow):
 
     stack_titlebar = GtkTemplate.Child()
     stack_perspectives = GtkTemplate.Child()
+    primary_menu = GtkTemplate.Child()
 
     def __init__(self, ratbag, *args, **kwargs):
         """Instantiates a new Window.
@@ -128,6 +129,7 @@ class Window(Gtk.ApplicationWindow):
         self.stack_titlebar.add_named(perspective.titlebar, perspective.name)
         if perspective.can_go_back:
             self._perspective_add_back_button(perspective, ratbag)
+        self._perspective_add_primary_menu(perspective)
 
     def _present_welcome_perspective(self, devices):
         # Present the welcome perspective for the user to select one of their
@@ -186,3 +188,15 @@ class Window(Gtk.ApplicationWindow):
         perspective.titlebar.add(button_back)
         # Place the button first in the titlebar.
         perspective.titlebar.child_set_property(button_back, "position", 0)
+
+    def _perspective_add_primary_menu(self, perspective):
+        hamburger = Gtk.Image.new_from_icon_name("open-menu-symbolic",
+                                                 Gtk.IconSize.BUTTON)
+        hamburger.set_visible(True)
+        button_primary_menu = Gtk.MenuButton.new()
+        button_primary_menu.add(hamburger)
+        button_primary_menu.set_visible(True)
+        button_primary_menu.set_menu_model(self.primary_menu)
+        perspective.titlebar.pack_end(button_primary_menu)
+        # Place the button last in the titlebar.
+        perspective.titlebar.child_set_property(button_primary_menu, "position", 0)