New tab should be placed after the current tab

Summary:
When opening a new using the context menu the new tab should be placed
after the currently open tab, not at the end of the tab list.

BUG: 403690

Test Plan:
Open multiple tabs and select a different tab than the last one.
Open a new tab using the context menu and see that it opens after the currently
selected tab.
Open a tab using Ctrl+T and see that it opens at the end.

Reviewers: #dolphin, ngraham, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: elvisangelaccio, ngraham, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D18945