skip forbidden pages and distinguish them from empty selections

Collected pages the user may not read are dropped from the export and remembered
separately, so a selection where every page is forbid

skip forbidden pages and distinguish them from empty selections

Collected pages the user may not read are dropped from the export and remembered
separately, so a selection where every page is forbidden can be told apart from
one where nothing was selected and reports "no read access" instead of "nothing
selected". The forbidden message no longer lists the page ids or references the
removed 'Skip Forbidden Pages' option; its outdated translations are dropped so
they get re-translated.

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have collectors check for page existance

extract $INPUT dependencies out of Collector classes

This should make tests and a future command line interface easier to
implement.

more refactoring

The basic handling of page collecting, caching and rendering is now
defined.

There is still some ugly dependency in the renderer and the
ImageProcessor loading is not reimplemented

more refactoring

The basic handling of page collecting, caching and rendering is now
defined.

There is still some ugly dependency in the renderer and the
ImageProcessor loading is not reimplemented yet.

After the above a first attempt to actually run the code can be made.

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move page collecting into separate classes