Make the open url in tab feature a bit more discoverable

Summary:
Introduce a context menu item that invites to open the button under the menu in a tab.
It used to just work with the middle button, now it works with control+click as well, much like in web browsers.

BUG: 402073

Reviewers: #dolphin, ngraham, #frameworks, elvisangelaccio

Reviewed By: #dolphin, ngraham, elvisangelaccio

Subscribers: elvisangelaccio, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D17550