[examples] Add Image Viewer readme and screenshots.

Author: mpaperno

examples/imageviewer/README.md

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+## Image Viewer Example Application ##
+
+This is a simple but fully functional image viewer application with a "gallery view" page
+and another for viewing a full size image with adjustable scaling, rotation, and zoom factors. 
+This example is primarily meant to demonstrate usage of the [StackOverlayLayout][1].
+
+![Gallery view folder loaded](doc/imageviewer-grid_loaded.jpg)
+![Full image view with info overlay](doc/imageviewer-image_info.jpg)
+![Gallery with alternate theme](doc/imageviewer-grid_alt.jpg)
+![Full image view with alternate theme](doc/imageviewer-image_alt.jpg)
+![Gallery with info overlay](doc/imageviewer-grid_infobox.jpg)
+![Full image with left toolbar](doc/imageviewer-image_tbleft.jpg)
+
+[1]: https://mpaperno.github.io/maxLibQt/class_overlay_stack_layout.html#details
+
+-------------
+Copyright (c)2019 Maxim Paperno. All rights reserved.  
+This example is part of the maxLibQt project and is governed by
+the same licensing.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.