Add possibility to use hardware volume buttons to take a picture

On many mobile operating systems it's possible to take picture using
hardware volume buttons.
It's easier to hold the phone and take pictures this way. Additionally
it makes possible to use selfie stick (at least with jack plug).

To be able to handle volume buttons global system shortcuts, that
controls audio level has to be disabled.

gdk_toplevel_inhibit_system_shortcuts tells compositor to block system
keyboard shortcuts, when the camera app has focus. In all other cases
system shortcuts are working like before even if camera app is running
in background.

src/main.c

@@ -1395,7 +1395,12 @@ activate(GtkApplication *app, gpointer data)
         create_simple_action(app, "quit", G_CALLBACK(run_quit_action));
 
         // Setup shortcuts
-        const char *capture_accels[] = { "space", NULL };
+        const char *capture_accels[] = {
+                "space",
+                "AudioLowerVolume",
+                "AudioRaiseVolume",
+                NULL
+        };
         gtk_application_set_accels_for_action(app, "app.capture", capture_accels);
 
         const char *quit_accels[] = { "<Ctrl>q", "<Ctrl>w", NULL };
@@ -1496,6 +1501,12 @@ activate(GtkApplication *app, gpointer data)
 
         gtk_application_add_window(app, GTK_WINDOW(window));
         gtk_widget_set_visible(window, true);
+
+        // Inhibit system shortcuts to be able to use
+        // VolumeUp and VolumeDown hardware buttons to take a picture
+        GtkNative *native = gtk_widget_get_native (GTK_WIDGET (window));
+        GdkSurface *surface = gtk_native_get_surface (native);
+        gdk_toplevel_inhibit_system_shortcuts (GDK_TOPLEVEL (surface), NULL);
 }
 
 static void