Fix binding loop in pager

Summary:
Previously on height changed rowHeight changed which changes columnWidth
which in turn causes rowHeight to be re-evaluated - a loop.

Moving it to a state machine solves that using a similar design pattern
to the digital clock.

Test Plan:
Panel in horizontal looked normal
Panel in vertical looked normal
Moving panel back to horizontal still was fine

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2848