[FIX] compiler: fix xmlns attribute not being set correctly in firefox

[sdegueldre]

Firefox will not serialize the xmlns attributes of node inside
XMLDocuments, see https://bugzilla.mozilla.org/show_bug.cgi?id=175946
it will only output an xmlns attribute if the node being serialized has
a non-null namespaceURI.

When compiling templates, we currently use a special attribute,
"block-ns" to keep track of the namespace, but we do not make use of the
namespace to create the elements that will be serialized to the compiled
block string. This causes a difference in behaviour between Chrome and
Firefox: since we end up with an Element with two attributes: the
block-ns attribute and the xmlns attribute. Chrome will serialize both,
but Firefox will only serialize the block-ns because the Element does
not have a namespaceURI.

To fix this issue, we get rid of the block-ns magic attribute
completely, and use xmlns instead. We also use the namespace when
creating intermediate elements that will be used to create the
serialized block string: the namespace is only used to create the
elements but not set as an attribute, as this would cause chrome to
serialize it twice, causing a duplicate attribute error when parsing it
further down the line.

When creating the template element for the block at runtime, the xmlns
attribute is used both as the namespace with which to create the
element, and set as an attribute, this doesn't cause issues when
serializing later because the namespaceURI is never serialized as an
attribute when serializing an HTML document 1, so we avoid the
double-serialization in Chrome, and when doing HTML serialization,
Firefox will correctly serialize the attribute.

It's desirable that the xmlns is set as an attribute to allow users to
use owl to render SVG, and then use the HTML serialization of it as a
SVG even outside the context of an HTML document (eg to generate an SVG
file or an SVG data URL).