#15 Remove singleton objects from GcApp

[Carlos Martín]

GcNetManager, GcSessiionManager and GcPluginManager are all singleton objects and aren't aggregated of GcApp. Remove from it.

Objects which requiredsuage of some of them should invoke:

session = g_object_new (GC_TYPE_SESSION_MANAGER, NULL)
{or}
session = gc_session_manager_create ()

[Carlos Martín]

Logged In: YES
user_id=260936
Originator: YES

session fixed on r645
GcPluginsManager and GcNetManager is pending