Issue 6328 - Moved all icons into skin/icons directory

This MR for #205 (closed) contains the following changes:

• Moved icons as outlined in issue description
• Updated references to icons
• Removed redundant "icon-" prefix from updates page icons
• Fixed: Removed alt text for decorative Adblock Plus logo in desktop options page
• Fixed: Removed unused logos
  • abb-logo.png: This image was introduced in trac#3031 and should've been removed as part of the first-run page redesign.
  • abp-64.png: This image isn't used by anything in adblockplusui and is also not being imported by adblockpluschrome.