Update style

This patchset is to add the possibility to modify a style. Not only the name, description and removing some iop, but also by adding the possibility to update or insert an iop from the selected image.

In lighttable, when selecting a style and clicking [edit]:

- When no image are selected, there is no change in behavior.

- When an image is selected, the dialog has a new column named update which can be used to select the iop to update into the style. And iop from the selected image are proposed for inclusion.

Nothing fancy, but I had the need for this... so here it is.