Allow external paint servers

The SVG spec allows for, e.g., fill properties to have URLs pointing to a paint server (like an linearGradient element). Inkscape however only supports paint servers within the same file, that is, URLs starting with '#'.

For consistent workflows with many single SVG documents (e.g., SVG served over the web with respect to caching) it would be *very* useful, if Inkscape would allow this kind of cross-referencing SVG files. The idea, why it's important, would be for example to have one cachable SVG file with definitions of gradients and patterns and referencing that from various other SVG files.

This feature could be similar in terms of implementation efforts: #167936 "CSS support"

Bug appearing in: Inkscape 0.46 Win/Linux

How to reproduce:

1. Create an SVG file "server.svg" with a linearGradient and id="test"

2. Create a second SVG file "client.svg" with a rect element and fill="url(server.svg#test)"

Expected outcome:

The rect should be filled with the gradient from server.svg

Cheers,
Manuel