Sets our main struct and passes it to the parent class.
The ::close signal is a [keybinding signal]GtkBindingSignal which gets emitted when the user uses a keybinding to close the window.
The ::search signal is a [keybinding signal]GtkBindingSignal which gets emitted when the user uses a keybinding to start a search.
Get the main Gtk struct
the main Gtk struct as a void*
the main Gtk struct