Use GResource for menu XML descriptions

Instead of a hardcoded ugly long C string, use GResource support to build
the XML in the binary, with the added niceties of using resources (e.g.
gtk_builder_new_from_resource(), resource:// URIs, etc).
Enable the "Copy" action only when there is a selection

Enable/disable the "win.copy" action depending on whether a selection is
done in the VTE widget.
Show an icon in the header bar on terminal beeps

If the terminal window is active, the icon is shown and hidden after
three seconds. If the terminal is not active, then it will be displayed
and hidden when the window gets re-activated (following the window's
urgency hint).