Main window: export: display zoom when exporting

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: Export: Take into account the page format specified by the user

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Main window: Export: Partially fix weird behavior when opening the export dialog for the 2nd time

Signed-off-by: Jerome Flesch <jflesch@gmail.com>