Add header qtest_widgets.h, similar to qtest_gui.h, but for QApplication.

Add header qtest_widgets.h, similar to qtest_gui.h, but for QApplication.

The current alternative is to define QT_WIDGETS_LIB before including
qtest.h, but this is not convenient/intuitive when using other build
systems than qmake. If one forgets the define, crashes happen when
using QApplication-related code.

Use <QTestWidgets> in one of the widgets autotests, for testing.

Change-Id: Id96be4976723aea3e8a28c9d0d594daab25a6d90
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>