Change base class attribute visibility about aimeos-core

[aimeos]

"salt" is unused and deprecated. It will be removed in the next months. You can simply pass "null" in your class.

We don't use protected variables in Aimeos for better encapsulation and to avoid them being part of the interface, so they cannot be changed any more without breaking the class interfaces. There are only a few places where we thought we need them to be protected, but we regret that decisions sooner or later.

[aimeos]

The "salt" parameter has been removed now. Will be part of the 2017.01 release