Add support for alter table's design without losing data

[staniek]

It should be possible to modify the table's design without losing all it's data.

This feature is of top importance for Kexi.

Designing, implementing and maintaining this feature requires large investment, approximately 5 man-months. It involves writing unit and functional tests and caring about specifics of SQLite, PostgreSQL, and MySQL.

KDE Design page: http://community.kde.org/Kexi/Plugins/Tables/Simple_alter_table
KDE Bug: https://bugs.kde.org/show_bug.cgi?id=125253

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