First pass expose inventory item settings in XML (folder_view_item.xml)

Optimized LLFolderViewItem constructor by passing param block as
const ref to avoid copies.
Removed per-item arrow and background images, use defaults.
Removed unneeded static image pointers.
Removed hard-coded rounded_square.tga, replaced with Rounded_Square
Ditto for folder_arrow.tga
Reviewed with Richard.