(Bug 34383) Canonical namespace names

For more details, see [1].

https://bugzilla.wikimedia.org/show_bug.cgi?id=34383
Re-introduce enableSemantics() to avoid Notice: Use of undefined constant SMW_NS_PROPERTY

After smwgNamespaceIndex and $smwgNamespace are set enableSemantics() is
called to specify SMW related custom namespaces.

## Example LocalSettings
$smwgNamespaceIndex = 104;
enableSemantics( 'http://example.org/id/', true );
...
Some minor clean-up