Add startCapture method

Summary:
Starts capturing as if the user clicked on the main button. Useful if one
wants to start capturing upon other user input than clicking on the item.

Test Plan: Call the method

Reviewers: Frameworks, davidedmundson, broulik, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28699