Main screen updated to use linq

Working on adding products and other parts
Adding products, deleting groups, other.
Pretty much everything converted over

Need to fix cross thread exception on price imports with products in
groups
Fix for cross thread exception on price import

By clearing the selected group, and thus clearing all products from the
group product list, the cross thread exception does not get thrown when
updating products.

I don't really like this fix and I really want to find a better way, but
this works for now.