gobject: Box GMappedFile

GMappedFile is current unintrospectable, because it's not a registered
box type.  It already has reference counting functions, so there's
little reason not to box it.

This commit adds GMappedFile to the hoard of other boxes types handled
by gboxed.c

https://bugzilla.gnome.org/show_bug.cgi?id=712393
GDummyTlsBackend: cache the GDummyTlsDatabase

The default GTlsDatabase is supposed to be a singleton (and libsoup
has tests that will fail if it's not).