Fix wrong logic in "Next/Previous Tool View"

Summary:
Old version has broken logic:

1. Select any element from Left/Right dock (Tool View)
2. Select "Window->Next Tool View"
3. As a result previous element will be shown (elements are switched from bottom to top).

Test Plan: Tested on master and 5.0 branches.

Reviewers: apol, KDevelop

Reviewed By: apol, KDevelop

Subscribers: brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2926