* ui/mainwindow.glade (toolbar): Make glade-2 replace its old-style
magic "class names" with proper GtkToolItem and derivated elements.
Note that you'll need to apply the patch for #142761 in order to
make changing the toolbar style at runtime work.
(combo_pattern): Replace deprecated GtkCombo with GtkComboBoxEntry.

* src/controller.cc (ControlItem::add_widgets): Allow for both
Gtk::ToolButton and Gtk::Button widgets, and detect at runtime
which type to use.

* src/mainwindow.cc (MainWindow::load_xml): Due to #142755, we
currently have to use get_child() on combo_pattern to gain access
to the entry widget.  Also, a hack was necessary to attach the
tooltip to the entry without instantiating another Gtk::Tooltips
object.

* src/mainwindow.h: Remove a couple of unused forward declarations.