main: Add 'M' shortcut to toggle mute

'M' was already used for the (increasingly rare) "go to root menu of a
DVD", so mute is only toggled when not playing a DVD.

This is the same shortcut as YouTube uses:
https://support.google.com/youtube/answer/7631406?hl=en

Closes: #457

README

@@ -29,6 +29,8 @@ Ctrl+H:
 	Shortcut help
 
 Undocumented shortcuts:
+M (when playing a DVD):
+	Menu
 Ctrl+Q:
 	Quit
 Ctrl+W:

data/shortcuts.ui

@@ -62,6 +62,13 @@
                 <property name="title" translatable="yes" context="shortcut window">Decrease volume</property>
               </object>
             </child>
+            <child>
+              <object class="GtkShortcutsShortcut" id="toggle-volume">
+                <property name="visible">1</property>
+                <property name="accelerator">M</property>
+                <property name="title" translatable="yes" context="shortcut window">Mute/unmute</property>
+              </object>
+            </child>
           </object>
         </child>
         <child>

src/totem-object.c

@@ -3469,7 +3469,10 @@ totem_object_handle_key_press (TotemObject *totem, GdkEventKey *event)
 		break;
 	case GDK_KEY_M:
 	case GDK_KEY_m:
-		bacon_video_widget_dvd_event (totem->bvw, BVW_DVD_ROOT_MENU);
+		if (totem_playing_dvd (totem->mrl))
+			bacon_video_widget_dvd_event (totem->bvw, BVW_DVD_ROOT_MENU);
+		else
+			totem_object_volume_toggle_mute (totem);
 		break;
 	case GDK_KEY_AudioNext:
 	case GDK_KEY_Forward: