always connect to "Local" source

Summary:
We need Local to be *always* present, even if not disaplayed as
it's used for formatting in ToolTip.dateTimeChanged()
this makes sure local is always in the connected sources, even if
it's not explicitly shown to the user

Test Plan:
tested, local timezone is not shown in clock/tooltip when
selected, and no warnings from dateTimeChanged()

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2936