Previously, when an image was selected as wallpaper it would be copied unconditionally into the "Wallpapers" directory. The image would be copied even if it was already in the "Wallpapers" directory, resulting in copies of the image being created in the same directory. This made it incovenient to potentially store multiple backgrounds in the "Wallpapers" directory and switch between them due to the fact that each wallpaper change would result in the selected image being copied. To avoid this, introduce a check, and only copy the image if its parent is not the "Wallpapers" directory (as determined by `g_file_equal()`). For local files, `g_file_equal()` only compares the absolute paths - at least at the moment. Comparing paths is not enough to decide whether two entities are really the same in the presence of e.g. bind mounts. However, such configuration is arguably a very rare (if not non-existent). Nonetheless, users should not experience any regressions even in that case since when `g_file_equal()` returns a false negative, the old behaviour applies.