Add better support for GtkTemplate

This is a feature of GtkWidget and GtkBuilder. If unfamiliar https://blogs.gnome.org/tvb/2013/04/09/announcing-composite-widget-templates/

Here is something I got working but is less than pretty:

local lgi = require('lgi')
local GLib = lgi.require('GLib')
local GObject = lgi.require('GObject')
local Gtk = lgi.require('Gtk', '3.0')

local Test = lgi.package('Test')

function template_connect_function(builder, object, signal_name, handler_name,
                                   connect_object, flags, class_name)
	local instance = builder:get_object(class_name)
	assert(instance ~= nil)

	local handler = instance[handler_name] -- TODO: Readable error
	assert(connect_object == nil) -- TODO

	-- FIXME: Need a closure around handler with the correct first arg, and extra data
	-- TODO: This is ugly, should have more direct approach
	object['on_' .. signal_name:gsub('%-', '_')]:connect(handler, nil, flags == GObject.ConnectFlags.AFTER)
end

-- Window ----------------
local ui = [[
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="TestWindow" parent="GtkApplicationWindow">
    <property name="title">Title!?</property>
	<child>
      <object class="GtkButton" id="button">
        <signal name="clicked" handler="on_clicked"/>
        <property name="visible">1</property>
      </object>
    </child>
  </template>
</interface>
]]

Test:class('Window', Gtk.ApplicationWindow)

function Test.Window._class_init(name, class)
	Gtk.WidgetClass.set_template(class, GLib.Bytes(ui))
	Gtk.WidgetClass.bind_template_child_full(class, "button", true, 0)
	Gtk.WidgetClass.set_connect_func(class, function (builder, object, signal_name, handler_name,
                                                      connect_object, flags)
		template_connect_function(builder, object, signal_name, handler_name,
                                  connect_object, flags, tostring(name):gsub('%.', ''))
	end)
end

function Test.Window._init(instance)
	Gtk.Widget.init_template(instance)
	instance.priv.button = instance:get_template_child(instance._type, "button")
end

function Test.Window:on_clicked(button)
	print('clicked')
end

-- Application ------------
Test:class('Application', Gtk.Application)
function Test.Application:do_activate()
	self._type._parent.do_activate(self)
end

function Test.Application:do_startup()
	self._type._parent.do_startup(self)

	local win = Test.Window({application=self})
	print(win.title)
	win:present()
end

function Test.Application:do_shutdown()
	self._type._parent.do_shutdown(self)
end

-- Main ------------
local app = Test.Application()
app:run()

Start with the GtkTemplate example in the issue and review the GtkWidget and GtkBuilder APIs it references. Compare the current workaround for template children and signal handlers with the desired direct support; done should cover template setup, child binding, and signal connection without the shown custom helper.