Introduce an Info Center mode

Summary:
withthe command line switch -i, start
systemsettings loading the KInfoCenter modules and hyerarchy instead.
* KAboutData and dbus service name will be the same of KInfoCenter
* The startup page is not shown but the "System Information" kcm is loaded instead
* KACtivityStats is disabled
* The home button will load the "System Information" kcm as well
* It will *always* be in sidebar mode regardless how systemsettings is configured
* Config dialog can't be open.

BUG:412975

Test Plan:
* SystemSettings mode functionality didn't change
* infocenter mode is fully functional

{F7930943}

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: meven, davidedmundson, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26895