This patch makes database support in KMyMoney optional instead of mandatory. I believe most users use XML storage, so they could switch off SQL storage  to save startup time and memory. Developers will benefit too, because they can switch off building of this plugin and save compilation time.

Following safty checks were implemented:
1) user tries to open database with SQL storage plugin switched off,
2) user opens database and switches SQL storage plugin off and tries to save his storage

I think that in future we could lower our core dependencies by droping off Qt5SQL.
I also think, that we could switch between XML and SQL on the fly.

Differential Revision: https://phabricator.kde.org/D10406