Make KF5Purpose dependency optional

Purpose dependency drags in glib packages, such as libaccounts and libsignon.

Purpose is also optional in Plasma, Okular, and Spectacle.

Test Plan:
Tested with both purpose-devel dependency installed and not installed
(only Share > Sent via Email... was tested).

Reviewers: Gwenview, ngraham

Reviewed By: ngraham

Differential Revision: https://phabricator.kde.org/D22251