Improvements to facsimile full page view

[simongray]

• Dorte only uses +/- to resize content, which results in the maximised facsimile to retain its size.
  • This should also work for people like Dorte who don't use touchpad gestures.
• Contrast slider for hard-to-read facsimiles.

[simongray]

With the changes to the illustration full page view, images with a horisontal aspect ratio do not work properly, e.g.: http://localhost:9000/app/reader/acc-1992_0005_094_Principes_0210-tei.xml

This must be fixed before this is done.