Filter out duplicate plugins

Summary:
If purpose is installed both as distro package and in a custom prefix,
we don't want to show each plugin twice. This patch filters out
duplicate plugins based on their pluginId.

Test Plan:

• Apply D13124 and make sure each purpose plugin shows up only once in the 'Share' context menu.

Reviewers: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13308