gobject: Add G_TYPE_FLAG_DEPRECATED

This can be used to mark entire types as deprecated,
and trigger a warning when they are instantiated
and `G_ENABLE_DIAGNOSTIC=1` is set in the environment.

There's currently no convenient macros for defining
types with the new flag, but you can do:

```c
_G_DEFINE_TYPE_EXTENDED_BEGIN (GtkAppChooserWidget,
                               gtk_app_chooser_widget,
                               GTK_TYPE_WIDGET,
                               G_TYPE_FLAG_DEPRECATED)
...
_G_DEFINE_TYPE_EXTENDED_END ()
```

Includes a unit test by Philip Withnall.

docs/reference/gobject/gobject-sections.txt

@@ -18,6 +18,7 @@ G_TYPE_IS_DERIVABLE
 G_TYPE_IS_DEEP_DERIVABLE
 G_TYPE_IS_INTERFACE
 G_TYPE_IS_FINAL
+G_TYPE_IS_DEPRECATED
 GTypeInterface
 GTypeInstance
 GTypeClass