Implement syntax highlighting in diff editor

All Qt Creator's main highlighters are used in the first place,
for other mimetypes generic highlighter is used as a fallback.

Task-number: QTCREATORBUG-9580

Change-Id: I863b9085520e5bdda142ce88f2074afeacee0531
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>

src/plugins/pythoneditor/tools/pythonhighlighter.cpp

@@ -65,9 +65,20 @@ using namespace PythonEditor::Internal;
  * @endcode
  */
 
+PythonHighlighter::PythonHighlighter(QTextDocument *parent) :
+    TextEditor::SyntaxHighlighter(parent)
+{
+    init();
+}
+
 /// New instance created when opening any document in editor
 PythonHighlighter::PythonHighlighter(TextEditor::BaseTextDocument *parent) :
     TextEditor::SyntaxHighlighter(parent)
+{
+    init();
+}
+
+void PythonHighlighter::init()
 {
     static QVector<TextEditor::TextStyle> categories;
     if (categories.isEmpty()) {