Save embedded images as attachments to note

... instead of inline base64, as it seems to be more appropriate.

• Use contentID to uniquely identify them in the note html code (i.e. <img src="cid:XXX@KDE" />);
• When saving, use RichTextComposerImages routines to obtain contentIDs and to replace img src to cid:...
• When loading, use QTextDocument::addResource to make QTextDocument recognize attachments