WIP: details-page: Add screenshot viewer

Currently there is no option to make the screenshots larger which often makes them hard to read.

To fix this situation, open a simple screenshot viewer after clicking on the main screenshot. It 'zooms' in, making the screenshot full screen with left/right arrows to flip between screenshots, and an X to close it and return to the application view. The navigation buttons show after motion and hide after a certain timeout, there is also support for key presses (arrows to move between screenshots, esc to close).

Note that this is very WIP, originally I created a class GsImagePage based on GrImagePage from gnome-recipes, but had some problems, you can see the code in my other branch. I like this approach, but wanted to upload a WIP version to get some feedback.

Some screenshots:

Closes #313