Use Plasma::PluginLoader to list applets
Summary:
PluginLoader has some additional checks for some obscure config options,
such as ExcludeCategories in plasmashellrc.
By doing the applet listing manually widget explorer skipped that.
Test Plan: Checked widget explorer still listed everything
Reviewers: Plasma, mart
Reviewed By: mart
Subscribers: mart, apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D2877