Fix build with libsigc++ 2.4+ (group was removed).
diff -Naur a/actions.cc b/actions.cc
--- a/actions.cc	2015-11-04 19:56:49.351107678 +0100
+++ b/actions.cc	2015-11-04 19:57:07.161246969 +0100
@@ -51,10 +51,8 @@
 	context->set_icon(pb, pb->get_width(), pb->get_height());
 }
 
-bool negate(bool b) { return !b; }
-
 TreeViewMulti::TreeViewMulti() : Gtk::TreeView(), pending(false) {
-	get_selection()->set_select_function(sigc::group(&negate, sigc::ref(pending)));
+    get_selection()->set_select_function(sigc::mem_fun(*this, &TreeViewMulti::negate_pending));
 }
 
 enum Type { COMMAND, KEY, TEXT, SCROLL, IGNORE, BUTTON, MISC };
diff -Naur a/actions.h b/actions.h
--- a/actions.h	2015-11-04 19:56:49.351107678 +0100
+++ b/actions.h	2015-11-04 19:57:07.161246969 +0100
@@ -30,6 +30,11 @@
 	virtual void on_drag_begin(const Glib::RefPtr<Gdk::DragContext> &context);
 public:
 	TreeViewMulti();
+    bool negate_pending(const Glib::RefPtr<Gtk::TreeModel>& model,
+                        const Gtk::TreeModel::Path& path,
+                        bool path_currently_selected) {
+        return !pending;
+    }
 };
 
 class Actions {