[refactor] Modify variable name (since it is  used internally in its class)
[refactor] Modify variable name  (onZoomIn -> _onZoomIn) since it is  used internally in its class
[refactor] Modify variable name  (onZoomOut -> _onZoomOut) since it is  used internally in its class
[refactor] Modify variable name  (onZoomFit -> _onZoomFit) since it is  used internally in its class
[refactor] Modify variable name  (onZoom100 -> _onZoom100) since it is  used internally in its class
[refactor] move self._dotwidget to __init__ ( separate widget create and layout)
[refactor] Use instance variable to store QAction.
[refactor]
[refactor] Modify variable name  (createLayout -> _setup_ui)
[refactor] move the setup of QAction to _create_action
[refactor] move the setup of connection to _create_connection.
[refactor] Use QMainWindow.
[refactor] use ' ' to constrcut string literal.
[Feature] (Open FIle / Exit Application]) Implement open dot file in the appliction.