Store / edit operation settings

[alukach]

A layer that is a product of an operation ran on one or many other layers should store how it was generated (the operation, the settings, and the source layers).

These settings should be editable. When edited, the operation is re-run and the layer regenerated (ie, I produce a layer by doing a 5km buffer on a file, I then decided I'd rather the buffer be 6km. Rather than generating a new layer, I should be able to alter the 5km buffer generated layer to be 6km and have it update automatically.

This is required to bring us toward the goal of Cascading Operations (#141), as outlined in #130.