Use Ctrl+Shift+, as the standard shortcut for "Configure <Program>"

Summary: FEATURE: Use Ctrl+Shift+, as the standard keyboard shortcut to invoke KDE programs' "Configure <Program>" menu items.

Test Plan:
This shortcut is not used by anything else. Searched on lxr, found one conflict in DigiKam, and the developers agreed to change it: https://bugs.kde.org/show_bug.cgi?id=386335

Will wait to land this until Digikam 5.8.0 is released to prevent any shortcut conflicts.

Tested in KDE Neon. Tried out Plasma, KWin, Dolphin, Kate, Konsole, Gwenview, Okular, Konversation, KTorrent, and Skanlite; all now have a consistent keyboard shortcut for their "Configure <Program>" menu items.

Reviewers: Frameworks, VDG, broulik, rkflx

Reviewed By: rkflx

Subscribers: ilic, abetts, elvisangelaccio, aacid, argonel, marten, graesslin, broulik, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8296