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

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: meven, davidedmundson, broulik, plasma-devel

Tags: Plasma

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