Database#tables and DBMLTable#columns should be read-only

[Spikhalskiy]

Currently, it's too easy to end up with an inconsistent Database entity by creating a column or table and appending it directly to Database#tables or DBMLTable#columns collections. Doing so, the correspondent table will not have a database property set, and the correspondent column will not have a table property set.

Database#tables and DBMLTable#columns should return read-only collections that don't allow direct modifications.