Add more tab access functions

Summary: These new functions to access the next and previous tabs are not used by anything yet, but it is envisioned that they would be useful for a variety of purposes--such as the "open path in new tab" feature from D11703

Test Plan: Dolphin still compiled and runs; new code is not actually used anywhere

Reviewers: Dolphin, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D12039