Add utility functions for ns creation
Add utility function to find namespaces
Add utility function to lazy-create ns
Lazy-create XML namespaces
Revert "Add dlna namespace only when it's actually used"

This reverts commit 50a5e45591e7bccb79250502c22233793dc6b2cb.

Conflicts:
	libgupnp-av/gupnp-didl-lite-writer.c
Add testcase for bgo705564#c12

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