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.

See owncloud/core#6475
Add decimal(precision,scale) column to unit tests
Typo fixed
Increment number of columns by one
Add unit tests for decimal type usage
Merge pull request #6476 from ogasser/dbal_precision_scale

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