Standard shortcuts: use Ctrl+PageUp/PageDown for prev/next tab.

Summary:
This is what other webbrowsers like Firefox and Chrome do,
and is certainly much more international than Ctrl+Comma/Ctrl+Dot which
is very qwerty centric.

Test Plan: Switching tabs in Konqueror

Reviewers: mdawson

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6553