[Purpose Menu] Create QQmlApplicationEngine on demand

Authored by broulik on Jan 23 2018, 1:34 PM.

Description

[Purpose Menu] Create QQmlApplicationEngine on demand

Loading the QML file accounts for 2% of Spectacle's startup time. Do it the first time it's actually done.

Differential Revision: https://phabricator.kde.org/D10034

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