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