Do not extract all template preview images, load from archives on demand

Summary:
Saves disk space, IO and avoids garbage preview files due to no
existing garbage collector.
Ideally description files would also not be extracted,
but KDesktopFile code sadly does not work against QIODevice.

Test Plan:
Previews work as before in the app wizard.
No direct idea for a unit test, unless extending API
of TemplatePreviewIcon just for that need.

Reviewers: #kdevelop

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5701