Remove Q_WS macros from qwidget-test.

Remove Q_WS macros from qwidget-test.

- Remove Q_WS-ifdefed sections and switch by platform name instead.
- Remove Q_OS_IRIX and Q_WS_QWS conditionals
- Fix deprecated QPixmap::grabWidget(), QPixmap::grabWindow()
  calls.
- Use QTest::qWaitForWindowShown() instead of
  qt_x11_wait_for_window_manager().
- Rewrite some platform-specific code using QPA interfaces.

Change-Id: I29ce4c4f3e5c05edddfd018dc2eabdd77738eb23
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>
Reviewed-by: Jason McDonald <jason.mcdonald@nokia.com>