gtkstack: allow templates to override all properties

gtkstack: allow templates to override all properties

Currently GtkStack has some G_PARAM_CONSTRUCT properties. That means,
the properties are set with its default value after the initializacion
of the object.
When using GtkBuilder to build objects, GtkBuilder creates them and
after that sets the properties found on the xml definition.
However, this is not true for templates because the template is initialized
in the init() function of the actual object, and after that, the construct
properties will be set.

This is a problem when someone wants to use templates with GtkStack and
set those properties, since they will be set on the tempalt initialization
and set again to its default values afterwards.

To fix this, make those properties not G_PARAM_CONSTRUCT.

https://bugzilla.gnome.org/show_bug.cgi?id=758086