Reuseable functions for handling of inherited pks

Two new functions on `contrib.sql.tools`:

  - `is_inherited_primary_key(prop)` : Returns True if the `ColumnProperty` **prop** is an inherited primary key. Occuring Exceptions are not handled but raised. *Should* work with arbitrary levels of inheritance.
  - `get_column_for_current_model(prop)` : Return the `Column` that belongs to the model of the `ColumnProperty` **prop**. So if the ColumnProperty is for the parent, the Column for the parent ist returned. If the ColumnProperty if for the Child, the Column for the child is returned. Works with arbitrary levels of inheritance.

`AdminModelConverter.convert()`  and `ModelView.scaffold_list_columns()` are adapted to use the new functions.