Generalize function for visualizing whitespace from CppHighligher

Move function which formats all non-whitespace with a given format, and
all whitespace in the same range with correct whitespace highlighting
(merged with the non-whitespace format), from CppHighlighter to
SyntaxHighligher.

Change-Id: I8cac306f6362e804698068a0df0292f88726264f
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/syntaxhighlighter.h

@@ -85,6 +85,8 @@ protected:
     QTextCharFormat format(int pos) const;
 
     void formatSpaces(const QString &text);
+    void setFormatWithSpaces(const QString &text, int start, int count,
+                             const QTextCharFormat &format);
 
     int previousBlockState() const;
     int currentBlockState() const;