UI to configure properties at runtime

[nthiery]

Let the user drag an argument-free method from the method list up into the property list; from now on, this property will be displayed for any object in the same class.

Ideally:

• This would be persistent across Sage-Sessions
• Alternatively, there would be a way to store/export the current configuration

Variants:

• next to the run button of the method, there could be an add to properties button; a simpler to implement gesture
• the property widget could have a Add property or + button letting the user chose the property name from a drop-down with automatic completion, and possibly the class. And some similar UI for removing properties.