Improving IsDirty after switching modes and undo

[alfonsoml]

This is a remainder of the issues described in #117

Now the IsDirty works fine while the user doesn't switch modes, but if the user does some change, switch mode and then later on he reverts to the original code the IsDirty will remain as true.

This isn't really a big issue, but it's better to leave it documented here.

[fredck]

Just to precise. A problem happens only in the following case:

1. Load sample08.html ( IsDirty() = false )
2. Switch to Source ( IsDirty() = false )
3. Make changes to the text, even a single char ( IsDirty() = true )
4. Switch back to WYSIWYG ( IsDirty() = true )
5. Undo as much as possible to return to the startup state ( IsDirty() = true, which is wrong. It should be false )

[fredck]

#638 is a DUP.