revert: restore system default window instead of custom one.

custom window is not the goal of this project.
we remove it and use system default window instead.

app/framelesswindow.h

@@ -18,23 +18,8 @@ public:
     explicit FramelessWindow(QWidget *parent = nullptr);
 
     void setCentralWidget(QWidget * widget);
-    void installResizeCapture(QObject* widget);
-
-protected slots:
-    void showEvent(QShowEvent *event) override;
-
-protected:
-    bool eventFilter(QObject *o, QEvent *e) override;
-    bool mouseHover(QHoverEvent* event, QWidget* wg);
-    bool mousePress(QMouseEvent* event);
 
 private:
-    Qt::Edges m_oldEdges;
-    Qt::CursorShape m_oldCursorShape;
-
-    Qt::CursorShape getCursorByEdge(const Qt::Edges& edges, Qt::CursorShape default_cursor);
-    Qt::Edges getEdgesByPos(const QPoint pos, const QRect& winrect);
-
     QVBoxLayout * m_centralLayout = nullptr;
     QWidget * m_centralWidget = nullptr; // just a pointer, doesn't take the ownership.
 };