Doc: Updated thumbnail images for some examples.

-Application Example
-Analog Clock
-Analog Clock Window
-Scribble
-Digital Clock

-Thumbnail images were created with an obscure window manager and
they look out of place in the Welcome Mode in Qt Creator.
-Used Windows 7 as the platform.
-Images within the documentation not updated because they are still
relevant.

Task-number: QTBUG-33597
Change-Id: I255fc4960e9adcec273f21287ef2182656c6b007
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>