DB: Support DECIMAL(precision,scale) syntax in XML

Add support for specifying the precision and scale of a decimal data
type to the XML description language. Added new unit tests and adapted
existing ones.

See owncloud/core#6475

Backported from owncloud/core#6476
Merge pull request #6666 from ogasser/decimal_precision_scale_stable6_bp

DB: Support DECIMAL(precision,scale) syntax in XML