EHTMLEditorView - Preserve empty lines when processing the text in convertor

Also insert the BR elements instead of the non-breaking space as a content to
empty paragraphs to avoid unwanted character there.
EHTMLEditorView - Process the element only if it has children
EHTMLEditor - Stop using deprecated gtk-stock items
Bug 731507 - [webkit-composer] paste does not work, shift-ins or CTRL-V works

Together with previous commit that removes the usage of deprecated
gtk-stock items and unregistering the Ctrl+v shorcut this solves the
issue. When the action is triggered and WebView doesn't have focus we have
to focus it to insert the clipboard content there.
Bug 732202 - [webkit-composer] Paste scrolls view

We have to check if the caret is inside the viewport when we are trying to
scroll to it. If it is in the viewport we won't scroll.
Bug 731508 - [webkit-composer] no option to paste as text (without formatting)
EHTMLEditorActions - Hide HTML actions in plain text mode instead of making them insensitible
EHTMLEditorView - Fix clipboard paste into quoted content

When pasting the text from clipboard into the quoted content in the composer
paste it into it and not into the body.