'Set as Wallpaper' option for KDE

[Rudraksh88]

Added an option to set the current image as wallpaper for KDE Plasma.

[jurplel]

Couple of notes:

• The blast radius of this PR is very large—I think there may have been an issue basing the changes off of our current master.
• There should probably be a compile-time option to disable linking with dbus and this feature, or it should at least be enabled only on linux
  • An effect of that is that we might want to try to move this new KDE-specific code somewhere more isolated—the codebase is littered with ifdef like unexploded ordinance
• Are environment variables sufficient to determine if we are running KDE? Has it been investigated if checking for running processes is a more durable/practical solution (genuinely not sure)?