This task encapsulates the conclusion reached in D29798, which aimed to make it easier for people to figure out how to change the wallpaper in the lock screen and login screen while changing their desktop wallpaper. This is something that comes up frequently; see https://bugs.kde.org/show_bug.cgi?id=367666 and its duplicates. The following idea shook out:

We make a new KCM in System Settings called "Wallpaper". This KCM essentially displays the current desktop wallpapers UI with one exception: when you click Apply, it displays a sheet allowing you to select what you would like to apply the wallpaper to. When there is only one containment (common case), the following apply options are shown:

• Apply to desktop
• Apply to lock screen
• Apply to login screen (only enabled when the chosen wallpaper is a still image)

When there is more than one containment, the following options are shown:

• Apply to desktops (shows a grid view of all containments; the user would select the ones they want to apply the wallpaper to; has a "select all" button)
• Apply to lock screen
• Apply to login screen (only enabled when the chosen wallpaper is a still image)

Each option would each be checkable, so you could apply the wallpapers to everything all at once if you wanted to.

The separated wallpaper chooser UIs in the Lock Screen and Login screen KCMs could then be removed and replaced with links to this KCM. The current per-containment wallpaper chooser accessible from the desktop context menu could also perhaps be replaced with an embedded version of this KCM--or not, if people think that would be too annoying.

Thoughts?