Remove dependency on okularpart from the mobile components

Summary:
This only depended on okularpart due to the export header used in the
kcfgc file, so use a dedicated one for the mobile components that doesn't
export anything.

This enables disabling the part build for mobile-only builds such as
Android, and thus removing the need for some difficult dependencies
like KParts.

Reviewers: aacid

Reviewed By: aacid

Subscribers: aacid, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D19037