Solid color wallpaper support

• Adds new color slot wallpaper, defaults to a slightly darker bg (panel background color)
• Redmond XX and XX-SE updated to feature my favorite color as wallpaper.
• Redmond XXI updated to feature wallpaper color same as Win2000/Me default color (#396ba5)
• Small fix for navbar not properly applying topBarLink color
• Small refactor to how background image is applied
• Made it so that wallpaper is horizontally centered and top-aligned (alongside navbar)
• Added an option to hide instance-defined wallpaper if it's present and if user doesn't have their own wallpaper set up.