Import kget plugin

With the KParts method being deprecated, we need to import the plugin in Konqueror. Because this plugin only onteracts with kget using DBus, we don't have any dependency issues.

To avoid conflicts for package managers, the installed "konqueror_" prefix is added to the installed plugin. The datafiles have been renamed from "plug_in" to "plugin", which is what most people would expect anyways.

Task: https://phabricator.kde.org/T13806

@dfaure @stefanocrocco

Counterpart of kget!20 (merged)

Draft for now, since translations should be copied when it gets merged :)