Declarative dialog improvements for the non-Window use case

Platforms like Android and EGL don't support multiple top-level
windows, so we have to avoid trying to use widget-based dialogs
(because a widget dialog on top of a scene graph will result in a
second window), allow the QML dialog to be an Item, and decorate it
to look like a window.

Task-number: QTBUG-31898
Change-Id: I9af049f3265188e8be677a05a8bc6d1699b4cd00
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
photosurface: add C++ boilerplate launcher; open button

Creator refuses to build it for Android otherwise, and this is a good
demo for tablets.  Also added a button to re-open the FileDialog.

Task-number: QTBUG-31898
Change-Id: I9f90de316fcaa2679484f8a875737ffafa774f31
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>