Bug 7500 [FOLLOW-UP][Missing images] Use CSS Sprites for faster page loading

Bug 7500 [FOLLOW-UP][Missing images] Use CSS Sprites for faster page loading

- Adds shelf browser navigational arrows to the main sprite.
- Removes "caret.gif" (a sort of breadcrumbs separator in the OPAC) and replaces
  it with a similar Unicode arrow entity.
- Replaces item-bullet.gif (a custom list bullet used in the Cart) with data URI
  encoding the image in the CSS

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>