gobject: Use local variable names in macros less likely to clash

At least GDBus had code that had "_object" as a parameter to one of
its functions, but this clashes with the GObject macro
G_OBJECT_WARN_INVALID_PROPERTY_ID() which created a local "_object".

Since many of us cargo cult around copies of objects, let's be
defensive here and use local variable names less likely to clash with
application code.

https://bugzilla.gnome.org/show_bug.cgi?id=689377