Backend: PDF documents: Fix export as a PDF (aka copy)

Signed-off-by: Jerome Flesch <jflesch@wyplay.com>
Main window: don't try to display the doc or the page if it's already displayed

Signed-off-by: Jerome Flesch <jflesch@wyplay.com>
Merge branch 'testing' into unstable

Conflicts:
	src/paperwork/frontend/mainwindow.py

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