Add dtpr to KReportUnit

Summary:
Move all inline functions to .cpp
Add operator= due to const member d
Add factor() to obtain the pixelConversion value

Test Plan: checked unit conversions in property editor for 3 different items in kreportexample

Reviewers: staniek

Reviewed By: staniek

Tags: KReport

Differential Revision: https://phabricator.kde.org/D3792