Add KPluginMetaData::supportsMimeType

Summary:
This will be useful to look up KParts using KPluginLoader::findPlugins.

I would bet most users of KPluginMetaData::mimeTypes() do it wrong
because they don't handle inheritance...

Test Plan: Test passes

Reviewers: arichardson, aacid, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12179

Differential Revision: https://phabricator.kde.org/D26137