Improve DBAL's query builder by adding a boolean query builder using a structured stack of arrays.

This can be used everywhere in an SQL query where boolean operations are used.
Currently, this can only be used in the WHERE and the JOINs' ON clause.

The main and general use-cases are:
To flexibly the build of the WHERE clause in SQL queries
To ease, simplify and prevent errors when doing SQL query editing by phpBB's extensions