Change foreground of PlainTextContentProvider

Hi Josh,

Quick info popups like the one used to display the text of parse errors on mouse hover will make the popup use the same border, background, and default plain text foreground as the SyntaxEditor.  This is done so that syntax highlighting within quick info popups will be the same as in the editor for readability purposes.

If you'd like the foreground to be different, you could use the HtmlContentProvider instead. 

Or alternatively, you could make a class that inherits PlainTextContentProvider and overrides its GetContent method to examine the TextBlock that is created by the base method, and set its Foreground property.