TextEditor: Consolidate document access functions.

There was document(), textDocument() and baseTextDocument().
Two should be enough...

Change-Id: Id9e41c8d857c5cb3269a9fce5ab594d34448c982
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/cpptools/cppcompletion_test.cpp

@@ -103,7 +103,7 @@ public:
         QStringList completions;
         CppCompletionAssistInterface *ai
             = new CppCompletionAssistInterface(m_editorWidget->document(), m_position,
-                                               m_editorWidget->baseTextDocument()->filePath(),
+                                               m_editorWidget->textDocument()->filePath(),
                                                ExplicitlyInvoked, m_snapshot,
                                                ProjectPart::HeaderPaths());
         CppCompletionAssistProcessor processor;