TextEditor: Simplify SnippetProvider handling

The group identifier is as good as the actual object, so use that
on the generator and the consumer side.

Change-Id: I6ccd54a2362631af0b50583f12563b8e41a5dd78
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppcodestylepreferencesfactory.h

@@ -40,7 +40,7 @@ public:
     QWidget *createEditor(TextEditor::ICodeStylePreferences *settings,
                           QWidget *parent) const;
     TextEditor::Indenter *createIndenter() const;
-    TextEditor::SnippetProvider *snippetProvider() const;
+    QString snippetProviderGroupId() const;
     QString previewText() const;
 };