When changing the composer mode from HTML to Plain Text avoid reloading of the web view

Also preserve the selection (if it is possible) when changing the mode.
Fix handling of the selection in the composer when saving the message draft

When saving the draft don't lose the active selection in the web view.
Also restore the selection when the draft is again opened. Also fix the
situations when the spell check was not activated when the composer was
opened.
EHTMLEditorView - Correctly remove all child nodes from the HEAD element

To avoid the critical warnings printed in the console.