Ctrl-Tab should work in "split-only" view as well

Summary:
The Ctrl-Tab and Ctrl-Shift-Tab were working only when multiple
tabs were opened, but it should work as well when only one tab
is opened but multiple splits are created within that tab.

Test Plan:
Open konsole with a single tab, then create several splits.
Use Ctrl-Tab and Ctrl-Shift-Tab to navigate between the splits
in chronological order.

Reviewers: #konsole, hindenburg, tcanabrava

Reviewed By: #konsole, hindenburg

Subscribers: gennad, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D20395