Instead of keeping all items in a boost::ptr_vector(), LIB_ITEMs are now
stored in an integer (KICAD_T) to LIB_ITEMS map.
The map allows to quickly access a subset of items of given type.
As the items are stored per type, there is no need to call
LIB_ITEMS::sort() to assure the correct drawing order. As a result,
libraries load faster.
To retain the old interface, there is a LIB_ITEMS_LIST wrapper for
the map, allowing the developers to access the items as if it was a flat
list-like structure.
Maciej Suminski