MacOS: isMaximized broken (regression), hence saveGeometry too

Run the sample. Resize the window with the mouse. Click inside the window -> Q_ASSERT is triggered because isMaximized is still true.

This is bad because the isMaximized state is saved with saveGeometry(). This means that in many cases, saveGeometry/restoreGeometry won't produce the expected result when non-maximized windows are to be restored.

This is a regression compared to 4.8