Add undo/redo/crop/scale/modify canvas buttons

Add undo/redo/crop/scale/modify canvas buttons when required. Since those are not always needed.
Since, for example, ksnip has those built-in.

example/main.cpp

@@ -36,6 +36,7 @@ int main(int argc, char **argv)
 	kImageAnnotator->setTabBarAutoHide(true);
 	kImageAnnotator->addTab(pixmap, QLatin1String("image1"), QLatin1String("image1"));
 	kImageAnnotator->addTab(pixmap, QLatin1String("image2"), QLatin1String("image2"));
+    kImageAnnotator->showControlsWidget();
 	kImageAnnotator->adjustSize();
 
 	QMainWindow mainWindow;

include/kImageAnnotator/KImageAnnotator.h

@@ -49,6 +49,7 @@ Q_OBJECT
 	void showScaler();
 	void showRotator();
 	void showCanvasModifier();
+	void showControlsWidget();
 
 public Q_SLOTS:
 	void loadImage(const QPixmap &pixmap);