tst_qthreadpool: fix memory leak

Task-number: QTBUG-22673
Change-Id: I2e554a2ad0119b5f9ce2d36e4720538af5ef6b82
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
QLineEdit: Extract Method shouldShowPlaceholderText()

The condition under which the placeholder text should be shown changed recently,
so refactor the code to centralize the condition.

It turns out that setPlaceholderText() and paintEvent() disagree on the condition
already.

Change-Id: Id193a9a138042ab0690082dcf634cf7487a5276e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Add test for default (style dependent) spacings

Task-number: QTBUG-35099
Change-Id: If19d3ccf7177dcbecef30b3ba1e16b022a3d54c4
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>