Improve API: KoReportData::data() -> create()

Summary:
from calligra 2.9 aaac84665edc

Simple API change. Needs additional change to Kexi

"Use KoReportData as const when it's enough" 7289e60fdf1e6bf4 (also to kexi.git)

Test Plan: Compile

Reviewers: staniek

Reviewed By: staniek

Projects: KReport

Differential Revision: https://phabricator.kde.org/D1337