EMsgComposer - Use underscore to split the variable name
EMailSignatureEditor, EHTMLEditorView - Don't insert unnecessary new line when editting the signature
EHTMLEditorView - Don't use separate WebView for converting the HTML message to plain text, but do it in situ

One reason for it is the WebKit2 port (the separation of UIProcess and
WebProcess -> this would be more complicated) and the another one was
performance. On complicated HTML test message the "Edit as new" action
with "Lose Formatting" answer could lead to nearly 30 second of
processing various things. Now the gap there is ~2 seconds.
EHTMLEditorView - Queue the load operations if the view is not ready

And process them when the view is ready. I just reintroduced the code
from https://git.gnome.org/browse/evolution/commit/?h=wip/webkit-composer&id=3de9a46ff406f29c537d6e4ec1e75c56f0b53bfe
that I removed in past.