Wallpaper API refactor, WallpaperItem

refactor Wallpaper api similar to PalsmoidItem

Plasma Wallpaper plugins root objects will have to be
instances of WallpaperItem, and their contextual actions
defined in a declarative way:

    WallpaperItem {
        contextualActions: [
            PlasmaCore.Action {...},...
        ]
    }

This removes completely KActionCollection from wallpapers