Frontend: Add a dialog to edit a page (no button connected yet)

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Page editing: Make it possible to select a part of the image

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Merge branch 'master' into img_edit
Page editing: ImgGripHandler: Emit the signal 'grip-moved' every time the user moves one of the grip

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Page editing: Show the resulting page as it's been edited

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Page editing: Apply the changes and save the image on disk

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Fix UTF-8 support

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

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