Make values converted to user-defined units for Double values

Summary:
Make values converted to user-defined units for Double values

Extra:

• handle all combination of (hasUnit) and (locale is C) cases
• round double to ints using QLocale::toString()
• add suffix to the editor spin box based on the "unit" option
• add suitable "Double cm" property to the example app

Test Plan: Try the "Double cm" property in the KPropertyExample

Reviewers: piggz
Differential Revision: https://phabricator.kde.org/D713
Fixes T1235