Fix KActionSelectorPrivate::insertionIndex()

Summary: Old version ignores InsertionPolicy::AtBottom enum value and produce behavior same as for InsertionPolicy::AtTop.

Reviewers: Frameworks, aacid

Reviewed By: aacid

Subscribers: aacid, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5329