QtQuick.Dialogs examples: use controls and layouts; visual tweaks

Make tab and button labels shorter so that they fit even on very-small-
screen devices (like iOS).
Make the content of some tabs scrollable.
Background color of the Font tab matches the others.
Use RowLayout to make baseline alignment of checkboxes and associated
text fields possible.
Use ColumnLayout because it works better inside a ScrollView.
Use Label instead of Text so that the renderType is consistent.

Change-Id: Ie2d07153532f35d108a0b6ad0bb4f10326f79d24
Task-number: QTBUG-41999
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>