hocr editor: page image should be in the background

[milahu]

clicking on the page in the center widget
does always focus the page image in the hocr tree

expected:
clicking a text node in the center widget
should focus the text node in the hocr tree

doubleclicking a text node in the center widget
should start editing the word text in the hocr tree

screenshot:

when i remove the "graphic" node (page image)
then i get the same issue with the page node:
no matter where i click in the center view, i can only focus the page node

possible solution:
probably the page image should be rendered as a background image
and "graphic" nodes should only be used for subsections of the page
with non-text (graphical) content

example hocr and image files are in #731

gimagereader version c05e8c8

part of #438