Slider image custom paths are lost when the settings form is submitted

Steps to reproduce:

• Enable ICE-BUSINESS and set as default
• Go to admin/appearance/settings/icebusiness
• Notice 'path to slider image' textfields all show '/sites/default/files'
• Check "Enable default slider"
• Upload an image for the slider
• Click "Save configuration"
• When the page reloads, click "Save configuration" a second time
• Notice 'path to slider image' textfields revert to '/sites/default/fiels'

Suggested solution

The attached patch reworks the slider functionality using a method similar to the logo and favicon uploads including:

• Adding validations for images and dimensions
• Paths relative to the configured public file path
• Use a render array to display the slider

@see system_theme_settings_validate()
@see system_theme_settings_submit()