Added helper func for swapping two avl_el objs

This function is ugly and a hack, but should work like a charm and
really fast, too.
Added function to delete an element from the avl tree
Removed todo: Rebalancing of subtree added
Outsourced deletion of node into recursive approach
Transformed: awp_avl_els() does now swapping by relinking
Fix: After we are swapping by relinking, we must delete the "found" node here
Adapted: Return the root node or NULL on failure