Kexi: Add sorting in Forms

Summary:
BUG:150372
FIXED-IN:2.9.11

Test Plan:
Open any form with nontrivial sortable data. Use sorting a->z, z->a.
Current record is unchanged but its index most likely changes, what can be observed in the record navigator.

Reviewers: piggz, wicik, wkosowicz

Subscribers: Kexi-Devel-list

Projects: KEXI, #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D881