gapplication: fix memory leak

g_variant_get_child_value() returns a new GVariant, which we must
free.

https://bugzilla.gnome.org/show_bug.cgi?id=710885
gapplication: don't emit warnings for DBus calls

If a broken (or malicious) remote client calls Open or CommandLine
on a GApplication that does not implement those, return a DBus
error instead of going through and then emitting a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=710885