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.cpp

@@ -0,0 +1,23 @@
+#include "framelesswindow.h"
+
+#include <QVBoxLayout>
+
+FramelessWindow::FramelessWindow(QWidget *parent)
+    : QWidget(parent)
+    , m_centralLayout(new QVBoxLayout(this))
+{
+    this->setWindowFlags(Qt::Window | Qt::FramelessWindowHint | Qt::WindowMinimizeButtonHint);
+
+    m_centralLayout->setMargin(0);
+}
+
+void FramelessWindow::setCentralWidget(QWidget *widget)
+{
+    if (m_centralWidget) {
+        m_centralLayout->removeWidget(m_centralWidget);
+        m_centralWidget->deleteLater();
+    }
+
+    m_centralLayout->addWidget(widget);
+    m_centralWidget = widget;
+}