EEditorWidget: Speed up the e_editor_widget_force_spellcheck
EEditorWidget: Add support for messages that have background image

Also toggle inline images when changing the composer mode and change
the cid uri to base64 data in namespaces.
EEditorWidget: Fix wrong logic that caused removal of composer content after caret
When generating the HTML code of the message write the additional data after the message

Before this change we were putting BR elements and also the credits
before the actual HTML code of the message. But this was wrong as when
WebKit was loading the given HTML code that looked like
<br>CREDITS<html>MESSAGE_CODE</html> WebKit parsed it like
<html><br>CREDITS</html><html>MESSAGE_CODE</html>. As no elements are
allowed outside of HTML root element WebKit wrapped them into another
HTML root element. Afterwards the first root element was treated as the
primary one and all the elements from the second's root HEAD and BODY
elements were moved from to the first one. Thus the HTML that was loaded
into composer contained the i.e. META or STYLE definitions in the body.
With this change we are putting and wrapping the elements behind the
message and afterwards moving them into the body. Also when we need to
cite the whole body of the message we're appending the span with
-x-evo-cite-body class to the message.
EEditorWidget: Refactor the quoting functions
EEditorWidget: Fix style and small bug in process_elements
EEditorSelection: Avoid treating the DOMText and DOMElement
EEditorSelection: Refactor the wrap_lines function
EEditorSelection: Return Paragraph - Normal block format for paragraps in blockquotes
EComposerPrivate: Save the information how the message is handled into BODY attributes
Rework the way how the HTML messages are converted to plain text

Also now the user is asked whether he wants to lose the formatting of
message when he is replying on HTML message or editting message as new
and he doesn't have the "Format messages in HTML" option set.