TextEditor: Update tab settings for all tab changes

Not just for selected documents. This also removes the need for
the separate setupAsPlainTextDocument function.

Change-Id: I51a04eebbad37a7598068ce6e157a53c1519464f
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/cpptools/cpppointerdeclarationformatter_test.cpp

@@ -103,7 +103,6 @@ public:
 
         // Open file
         auto textDocument = new TextEditor::BaseTextDocument;
-        textDocument->setupAsPlainTextDocument();
         textDocument->setId(Core::Constants::K_DEFAULT_TEXT_EDITOR_ID);
         TextEditor::BaseTextEditorWidget editorWidget(textDocument);
         editorWidget.setupAsPlainEditor();