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 c05e8c8c22e73b5f502f1f1df669aed1433c9a36

part of #438