You wrote
http://ramnet.se/~nisse/diverse/temp/inkscape_new_prefs_mockup4.png
[...] It would of course require some work to fit this into inkscape but i have done at least 50% already and i'm willing to do the other 50% as well.
Please do so! I don't have the impression anyone other than me is doing Gtkmmification right now, and I'm still at Document Properties [sic].
However, you are advised to subclass your dialog from Inkscape::Dialog::Dialog, which takes care of some signals, the transientizing, and the Close Button for you.
On the issue of the Close Button, its existence is controlled by the dialogs.showclose preference which, astonishingly, can nowhere be set in the program, so please add this to the Windows subpage.
When you have something to look at, I might steal the design/code for Doc Props which is HIG-noncompliant in 13 points at the moment, according to the latest bug report.
Regards, ralf