#402 Create XoopsModelAlter to facilitate module installation/update

[zyspec]

Modules need a way to alter database tables using methods similar to the database read/write/status model classes during module updates. Attached is a proposed alter class (XoopsModelAlter) that will allow modification of tables without the module having to create the SQL syntax. This will allow the underlying database access to occur without the module having to write the SQL 'ALTER TABLE' statements.

[zyspec]

After further thought maybe these (or similar) methods should be added to the XoopsDatabaseManager class. In any event, the functionality is needed in the core so that tables can be altered during module installation in a database agnostic way (MySQL, SQLITE, PGSQL, etc.)

[Michael Beck]

Please also see this request from Irmtfan:
http://xoops.org/modules/newbb/viewtopic.php?topic_id=76043