SVG icon inclusion support

[Zlobin Nikita]

Hello.
For introduction: appart to include other svg files by file name, there could 
be ability specially for icon files to refer to other icons, just like desktop 
launchers do it.

Some known icons contain elements, which itself may be found as icons - 
various arrows, dots, analog clock, etc. And now i would like to just refer to 
some generic download icon (some generic download manager like kget, gwget, 
etc), instead of to manually copy icon's contents. With such approach icons 
for more special applications could better follow to system icon theme.

Ideally in this case icon theme could contain special category, called like 
"library", "elements", "atoms"... specially and sometimes only for inclusion 
(though mentioned above arrows and analog clock could be icons itself, and 
even more - analog clock could include arrows, border, etc... :).

And about mentioned example with download manager: i'm doing gui for youtube-
dl (youtube-dl-applet on kde-apps), and i would like to combine download 
manager icon with some video-related icon.

And since referenced icon is not mandatory to exist (for not installed 
application), there could be order of fallbacks, always ending with embedded 
svg content for last resort.

Some svg editors, e.g. inkscape, allow to edit svg content directly in text 
format. I guess, icon references could be implemented by some special tag 
(could be compared with doxygen and gtkdoc comments, treated only by doc 
generation utils).

Did not learn honestly, is it possible in svg, but hope, someone else knowes. 
If not possible, probably it makes sence to push such proposal for svg format.

(or svg supposes ability to ignore unknown xml tags, used by more specialized 
stuff - again, just my guessment)