Change component shown on selection changes

Summary:
::activated depends on the mouse click behaviour - as double click
starts a rename this led to the global shortcuts KCM being unusable with
double click set.

This patch follows the selection chaging - which is more semantically
correct as we're not really activating an option in the left hand
sidebar, merely selecting it to show the correct sub-options.

It fixes keys up/down changing selection but also allows double click
mouse settings to work.

Test Plan:
Ran.
clicked once, got the correct sub-options.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

BUG: 368661

Differential Revision: https://phabricator.kde.org/D2865