We are not exposing RNA_ObjectBase in the 2.80 API.
Thus we can't have operators relying on it (e.g, CTX_data_visible_bases,
CTX_data_active_base, ...). Otherwise users won't be able to override
context for these operators.
This commit keep the CTX_data_.*bases() functions around so we don't
need to change the operators and potentially break things that late into
2.80. However as far as the Python scripters are concerned there is no
base to be overriden, ever.
That also simplify the guessing game addon developers have to play when
trying to override an operatori context. They still need to find whether an
operator requires editables, visibles, selected, ... objects. But at
least they don't need to find out whether the operators need base or