Manuscript viewer (First sketch)

This is *not* to be merged but to let you comment on the initial infrastructure.
I have:

- a dockable panel with a widget
    - a View(QScrollArea)
        - an AbstractManuscriptContainer with
            - `ManuscriptImageContainer` or a (nonexistent) PDF container

The container is a simple QWidget() containing a list of `ManuscriptPage()` items. These currently are simple labels but will hold more information on the manuscript page later.

In this first sketch a FileDialog will pop up on startup, where you can select one or more images to be displayed. Of course there will have to be more options and actions. Currently I'm only interested in comments about the general approach.