Do not strtoupper the sorting mode.
Simplify the sorting definition.

Instead of set custom SQL with "<cell> => <sql>" simply write "<cell>
<sql>". <sql> can simply ASC or DESC or any custom SQL that is evaluated
by the data provider.
Evaluate fields grouping flag, when evaluate sorting mode.

Evaluate the flag on a property definition and inherit it to the default
sorting and grouping.
Rename $propertyInformation to $groupAndSortingInformation.
Correct check for sort direction match in DefaultDataProvider.
Remove now obsolete call to strtolower().
Merge branch 'hotfix/sorting-and-grouping' into develop