Note editor: unify handling of attachments and encrypted areas

[d1vanov]

For historical reasons there are two distinct approaches to handling of attachments in QtWebKit and QtWebEngine based parts of note editor - the webkit part uses plugins API which doesn't exist for webengine so it uses some different techniques. This circumstance complicates the codebase and maintenance + I noticed that plugins API silently ceased to work when using Qt4 build in Plasma 5. So there should be exactly one simple way to handle things like attachments and encrypted text areas - the way currently used by QtWebEngine backend.