Launch plugin with argument, change signals

Summary:
The kcms/runners plugin is now able to launch a specific plugin using the --arg parameter of kcmshell5 and the deprecated signals have been changed.
And the QLatin1String has been replaced by the QStringLiteral Macro, because the open config method does not have an overload with the QLatin1String class.

Test Plan: You can just type "kcmshell5 plasmasearch --args Dictionary" and additionally to the KRunner configs it should open the plugin config

Reviewers: broulik, mart

Reviewed By: mart

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26738