Use Qt key names (e.g. Qt::SHIFT + Qt::Key_Left) for shortcuts

Summary:
While investigating https://bugs.kde.org/show_bug.cgi?id=410110, I found that the
keyboard shortcuts were being set using a non-standard method. This patch corrects
that and uses the standard Qt style.

I was hoping that this would fix https://bugs.kde.org/show_bug.cgi?id=410110, but it
does not.

Test Plan: Use Elisa in English; keyboard shortcuts still work

Reviewers: mgallien, astippich, Elisa

Reviewed By: astippich, Elisa

Differential Revision: https://phabricator.kde.org/D22718