Feature: Add file template for QObject with pimpl

Summary:
The current QObject template has all private members exposed
in the class, and there are reasons to use a pimpl variant
instead.
As the current file template logic does not allow to control
the number of files generated (like only generating the private
header if the user toggles a config option), the QObject pimpl
template is a complete separate template.

Both QObject templates share template snippets where useful.

Reviewers: KDevelop, kfunk, mwolff

Reviewed By: KDevelop, kfunk, mwolff

Subscribers: mwolff, kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4435