Deprecate Object.Connect

[diamondburned]

This issue tracks the progress for deprecating the (*glib.Object).Connect and (*glib.Object).ConnectAfter methods, reason being that they're not needed anymore with autogenerated signal methods (which are more performant and correct).

• Find a way to specify ConnectAfter
  • Consider generating ConnectAfter methods instead, but that might bloat up documentation.
  • Consider explicit flags: w.ConnectHide(0, func() {})
  • Evaluate if function flags is a better idea:

func (w *Widget) ConnectHide(f func(), flags ...externglib.ConnectFlags)

w.ConnectHide(func() {
    // Might look too ugly.
}, glib.ConnectAfter)

• Consider autogenerating Connect methods that allow eliding arguments:
  • Will mostly help easing the transition from traditional Connect methods.

func (w *Widget) ConnectHide()

switch f := getFunc().(type) {
case func():
    f()
case func(label string):
    f(label)
case func(label string, target *gdk.Drop):
    f(label, target)
}

• Add a Notify function for notify:: signals:

// Notify connects f to the "notify" signal for the given property. f will be
// called every time the property's value is changed.
func (o *Object) Notify(property string, f func())

• Deduplicate the callback for signals with the signature func()
  • These are super common, so doing this will remove lots of repetitive code.
• Remove pkg gobject-introspection
  • We don't need this once we stop generating _get_type() marshalers.