Namespace declarations not reloaded if "not used"

Whenever you add a namespace declaration within the
<svg> tag, it is ignored when reloading it (not when
saving it) if this namespace is not *explitely* used in
the SVG file.

By "not explicitely" used, I mean that the SVG file
does not declare any element with the namespace prefix
but such a prefix may be used within a text (of an
attribute or an element) as an XPath expression.

Example:

<svg
[...]
    xmlns="http://www.w3.org/2000/svg"
 [...]
    xmlns:dpi="http://www.eseo.fr/DoPI/2004/dopidom"
    xmlns:hid="http://www.eseo.fr/DoPI/2004/domino">

[...]
  <g
     id="exiter"
     dpi:component="sovigo.tools.Exiter :
[domino](//hid:sensor[@hid:name='key esc'])" />

</svg>

The dpi:component use an XPath expression that is based
on the hid namespace prefix. The namesapce declaration
for the prefix works fine, but the namespace with the
hid declaration is omitted whenever you reload the file...

In a general way, I think that Inkscape sould define
less specific rules and should respect as closer as
possible the semantic of the SVG recommandation (for
example, the attribute transform="translate(0,0)" is
automatically removed, the translate is recompute when
an x or y is specified, and so on...)