[DigitalClock] Fix layout and QML warnings

Summary:
Fixed issue when time zone configuration page was not scaling corrently. As a result two
srollbars were rendered.
Fixed few QML layout warnings and undefined references.

Test Plan:
Open Time zones configuration page and shrink window - two scrollbars are rendered.
Expected: table is scalled correctly and only one scrollbar in table is rendered.

Reviewers: #plasma, #plasma_workspaces, ngraham

Reviewed By: ngraham

Subscribers: ngraham, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24853