Fix preview for file templates with options: add vars with default values

Summary:
For templates which based display on variables filled by options,
the preview showed empty spots, making the templates look broken.
Adding the vars from the options with the default values fixes this.

Reviewers: apol, KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4384