Address: rename isIPv6() to getType()

In preparation for generalizing Address in the next commits
BIG REFACTORING -- TODO split up commit; describe changes in commit message
ARP: refactoring: ARP is now next to the IPv4 module not below it; see TODO below!

Motivation:
 - performance improvement (datagrams no longer go through ARP)
 - conceptually better (packets to PPP no longer flow through ARP)
 - more consistent with how IPv6 and NeighbourDiscovery are organized

TODO:
 - update NED docu of both ARP and IPv4!
 - proxyARP is now both in IPv4 and ARP??
 - refactor ARP gates: queueIn+ipIn/queueOut (all connecting to IPv4 though)
 - note: IPv4RoutingDecision is now only effectively used on the ip-->arp connection
added GenericNetworkProtocol & co.
added MultiNetworkLayer
added DYMO protocol
added GPSR protocol
added RIP protocol