Background: Support different images on different displays and independent random selection

Add a configuration rule that allows the user to specify how wf-background should be applied across multiple displays, if available.

Ideally, I'd like to be able to specify all existing configuration values on a per-display basis. Displays could be addressed in the order they are identified by the compositor (i.e., display 0, 1, etc.) or by wlr-randr output identifier if feasible. In my particular case, I'd like to be able to set both displays to select randomly from a given directory, and have that random selection be independent per monitor. If nothing else, additional displays could show the image at the randomly selected index + display number, so that images would be different if not entirely random.

Until recently, this was a feature of the software. (Specifically, when enabling random image selection, selection would be independent between multiple displays.) It has disappeared since I underwent the general upgrade of Wayfire and associated apps from 0.10 -> 0.11, so I suspect this may have been an unintentional functionality in the first place, or a consequence of upstream changes at GTK4/gtk shell layer.

Start by tracing wf-background's configuration handling and the random image selection path for multiple displays, then compare the behavior before and after the Wayfire 0.10 to 0.11 upgrade. The completed work should define how displays are addressed and allow image selection to be configured independently per display, with behavior specified for additional displays.