Copy chosen background art to LMMS config directory instead

### Enhancement Summary
Currently you can choose to have a background art file from within Settings. However if you then delete that file (e.g. you just downloaded it & then cleaned your Downloads folder), the next time it not show up.

#### Justification
Wouldn't it be better if the image file was copied in the directory that LMMS keeps its settings instead?
Kind of how Windows is copying your chosen wallpaper image in its own folder, so even if then you delete that image from the source folder it doesn't bork your wallpaper?

Start by locating the Settings code that stores the selected background art and the handling for LMMS's config directory. Trace how the image path is persisted, then verify that the background remains available after the original source file is deleted. The issue is complete when a selected image is copied into the config directory and still loads afterward.

Written by the indexing model from the issue text.