ui: Workaround Qt bug causing erratic scrolling

The maximumValue binding on the ScrollBar inside a ScrollView is updated
incorrectly, resulting in the scroll bar position changing in some cases
when contentHeight and originY are changed.

A proper fix for this is submitted upstream at:
https://codereview.qt-project.org/#/c/92401/
Change-Id I2a19e4c70096259ef7bbb5a00727ac767c4b8c57

This workaround accomplishes the same goal, by using private properties
to break the problematic binding and update its value manually at the
right time. If the upstream patch is applied, the workaround will no
longer have any effect.

Fixes #76