[RFC]: Split AttachmentControllerBase into a core class and a widget class
All the QObject only stuff has been moved to AbstractAttachmentControllerBase and AttachmentControllerBase inherit from it. This allow to use the class from a QML application while sharing the logic.
KMessageBox::sorry has been replaced by signal emmited by the abstract class and the base class listen to it to show the old dialog.
This is a RFC because the end goal is to make the ComposerViewer class not depend on QWidgets anymore, so it will need some similar changes to other classes. I want to make sure this change will be accepted after being cleaned up.