Introduce ecm_generate_qmltypes

Summary:
Introduces a function that allows to generate plugins.qmltypes files for our
qml plugins using qmlplugindump much like Qt does.

Test Plan: Introduced it locally on Purpose and it works

Reviewers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D5087