EHTMLEditor - Headers and credits are in wrong order when forwarding a message or using the inline reply

Mark the headers and citations whe inserting them and restore them on
the right position.
EMsgComposer - Remove the unneeded code

The new line after the signature when signature on top is used is now
handled in EHTMLEditorView.
EHTMLEditorView - Correctly replace the tab characters with spaces
EHTMLEditorView - Improve the code that is used for quoting the plain text blocks