Edit objects

[albfan]

once selected allow to modify any prop in https://lazka.github.io/pgi-docs/GooCanvas-2.0/classes/CanvasItemSimple.html#properties

Each item extend with its own properties.

Except really obvious (like fill-color using a gtkcolorchooser) allow to edit by strings. This is not about safe edit, just to be able to do a quick screencast with Akira