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/glsleditor/glslhighlighter.cpp

@@ -39,8 +39,19 @@ using namespace GLSLEditor;
 using namespace GLSLEditor::Internal;
 using namespace TextEditor;
 
+Highlighter::Highlighter(QTextDocument *parent)
+    : TextEditor::SyntaxHighlighter(parent)
+{
+    init();
+}
+
 Highlighter::Highlighter(BaseTextDocument *parent)
     : TextEditor::SyntaxHighlighter(parent)
+{
+    init();
+}
+
+void Highlighter::init()
 {
     static QVector<TextEditor::TextStyle> categories;
     if (categories.isEmpty()) {