chore: no longer based on QMainWindow

Since most of the features that QMainWindow offered will never be
used in this program. We don't need a statusbar, a menubar, etc.

app/framelesswindow.h

@@ -0,0 +1,25 @@
+#ifndef FRAMELESSWINDOW_H
+#define FRAMELESSWINDOW_H
+
+#include <QWidget>
+
+QT_BEGIN_NAMESPACE
+class QVBoxLayout;
+QT_END_NAMESPACE
+
+class FramelessWindow : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit FramelessWindow(QWidget *parent = nullptr);
+
+    void setCentralWidget(QWidget * widget);
+
+signals:
+
+private:
+    QVBoxLayout * m_centralLayout = nullptr;
+    QWidget * m_centralWidget = nullptr; // just a pointer, doesn't take the ownership.
+};
+
+#endif // FRAMELESSWINDOW_H