Add a way to export item icons

[dmorcellet]

For instance, with a contextual menu on item icons. Rationale: item icons are made of up to 4 layers of basic icons. These are merged to render the final item icon. Wiki editors would like to get item icons easily.

[dmorcellet]

Done. From the item icon found in the item description window, right click to get the "Save item icon..." menu. It will bring a file chooser. The default file name is the name of the icon (ID1-ID2....png). The chooser keeps track of the last directory used for exports.