EComposerPrivate: Fix style
EEditorWidget: Insert styled paragraph in plain text mode
EEditorWidget: Insert NL after text when generating plain text message

Also handle signature differently to avoid unnecessary NL around it and
refactor the process_elements function
EEditorWidget: Improve quote symbols in plain text mode

Allow to change the quote symbol (right now just through define) and
color the symbols dependant on level of citation.