Small improvements to the 'Export to PDF' feature

Summary:
* Capitalize "PDF"
* Add the action to the KMReaderMainWin too
* Set default filename to <subject>.pdf

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: yurchor, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D27793