KFileItem: isWritable use KProtocolManager for remote files

When a file is not local, check if we have a protocol available for its url and that we can use it to write.

This prevents paste action to be presented in context menus of non writable protocols: remote:/ recentlyused:/

We could supplement it with a KIO::Stat call, for when the protocol supports writing.

@ngraham @dfaure @ahmadsamir