Allow to modify/add new properties to multiple objects

There are times when one wants to add custom property
to multiple objects. For example, add new property
"speed=30" to few links or modify existing property
like "color=red" to selected objects.

Currently user can change or add property to only a
single object. This is very time consuming and error
prone when there are 100s' of objects.

Motivation to do this: Output of inkscape is xml, so
one can assign different properties graphically and
then parse xml to use it outside of inkscape.