Add break; statements in the generated switch code for model::data()

Summary:
Compiler complains about generated code with
warning: this statement may fall through
and is slightly right, as the code might fall through if a role is used
which is not catched for a column.

Reviewers: vandenoever

Reviewed By: vandenoever

Differential Revision: https://phabricator.kde.org/D15541