Improve 'Interpolate attribute in group' extension

Merged username-removed-934908 requested to merge Moini/inkscape:interp_att_ext into master

The extension can now interpolate namespaced attributes (e.g. rounding/randomness of stars). It makes use of the previously unused units option. Removes some duplication (debatable if that's really an improvement, though). It now works if one uses different color formats for start and end value. It now allows the comma as decimal separator (objects vanished previously). Fixes crash of extension when Other attribute is not specified. Improved the dialog a tiny bit. Improved whitespace.

(sorry for mixing whitespace corrections with contents... Please view diff with 'hide whitespace changes' activated)

Please let me know if you'd like me to make any changes.