Scaling Slide Miniatures

[LeonBein]

As a presentation creator, I want the slide miniatures to use the full width of the miniature viewport,
in order to use the resizing of the viewport as a way to make them actually smaller or zoom in.

Acceptance criteria:

• When Powersqueak is opened, all visible miniatures use the full width of their viewport
• When resizing the miniature viewport width, all visible miniatures adapt their width accordingly

Estimated effort: 8