* glib/glibmm/filelist.am: Add variantdbusstring.h and variantdbusstring.cc.
* glib/glibmm/variantdbusstring.[cc|h]: New files. String classes meant
only for Variants with D-Bus object paths or D-Bus signatures.
* glib/src/variant.[ccg|hg]: Add specializations for
Variant<Glib::DBusObjectPathString>, Variant<Glib::DBusSignatureString>
and Variant<std::vector<Glib::DBusObjectPathString>>.
* glib/src/varianttype.[ccg|hg]: Add VARIANT_TYPE_OBJECT_PATH_ARRAY.
* tests/glibmm_variant/main.cc: Add test_object_path().

Make it possible to create a composite Variant containing variant type
o (D-Bus object path) or g (D-Bus signature). Bug 785700