[Rest] Rename "category" field to "icon"

[spacemanspiff2007]

Currently the category field of an item is used to configure the icon of the item.
The field should be renamed to "icon" to better reflect what is actually happening.
With the next version being openHAB 5 it's a good time to make that change.

See docs on icon

[lolodomo]

Category is certainly also present in REST sitemap APIs, meaning any sitemap UI should probably be updated too.

[lolodomo]

I was wrong for sitemap API, looks like icon seems to be used.