(XMB) Background image improvements

[sonninnos]

Description

Due to endless complaints and confusion about the current logic behind XMB wallpaper images I decided to revise it a bit:

• Allowed images to take precedence over color themes, so no need to select "Background Image" theme anymore
• Changed the default image opacity to the same as color theme opacity, so that the result is consistent with core output in the background
• Added a sublabel clarification about the new logic

Also:

• Fixed pointless image loading on every in-between step when pressing Back at playlists
• Fixed preserving dynamic background image everywhere when video reinits
• Added menu context reset on dynamic background toggle, otherwise images will get stuck on disable if "Appearance" image exists
• Removed "Lakka advanced" flag from related appearance options
• Cleanups

Related Issues

#15878