Add utility functions for ns creation

https://bugzilla.gnome.org/show_bug.cgi?id=705564
Add utility function to find namespaces

https://bugzilla.gnome.org/show_bug.cgi?id=705564
Add utility function to lazy-create ns

https://bugzilla.gnome.org/show_bug.cgi?id=705564
Lazy-create XML namespaces

https://bugzilla.gnome.org/show_bug.cgi?id=705564
Revert "Add dlna namespace only when it's actually used"

This reverts commit 50a5e45591e7bccb79250502c22233793dc6b2cb.

Conflicts:
	libgupnp-av/gupnp-didl-lite-writer.c

https://bugzilla.gnome.org/show_bug.cgi?id=705564
Add testcase for bgo705564#c12

Signed-off-by: Jens Georg <mail@jensge.org>