Add full-screen presentation mode for presentation documents

[Stefano Verzegnassi]

We need to add a full-screen mode for LibreOffice Presentation document type.
This should make the app go full screen and show a black background.

C++ implementation:
Parts should be rendered through the LODocument::paintThumbnail method (consider to rename it to a more generic 'paintPart'), using a QQuickImageProvider.
When a paint is required through this provider, the paint should have an higher priority then the RenderEngine and the thumbnail provider.

QML implementation:
* Consider to cache next part/slide for a faster rendering on screen.
* We should provide also a setting (using Qt.labs.settings[1]) for optionally toggle a transition used for switching from a part/slide to another.
* While I'm not still sure if we should show on-screen controls at the bottom-left corner, we surely need to provide some basic hook for mouse, keyboard and touch inputs:
    - Mouse:
         - onClicked (left button): go next slide
         - onClicked (right button): show contextual menu (with an option to exit from full screen mode and the list of slides)
    - Keyboard:
         - left arrow: go prev slide
         - right arrow: go next slide
         - [esc]: exit from full screen mode
    - Touch:
         - swipe from left to right: go prev slide
         - swipe from right to left: go next slide
         - onPressAndHold: show contextual menu (with an option to exit from full screen mode and the list of slides)

[1]: http://doc.qt.io/qt-5/qml-qt-labs-settings-settings.html