support for foreign keys

[rdanitz]

As of now, foreign keys won't get treated differently.
In a first step, highlight foreign keys and provide information about affected tables and columns when a foreign key is selected.
Then, come up with a scheme how to treat selected foreign keys and either:
a) Indicate tables and columns which should be included in the selection based on foreign keys (and eventually prevent proceeding).
b) Automatically replace foreign keys and substitute respective columns (or reference them respectively).