Show and hide the menu bar with the 'alt' key

Only applicable for systems without a native menu bar (i.e. almost anything that is not macOS or Ubuntu Unity). On these systems, the menu bar appears on top of the window, which does not look good next to the tool bar already up there.

When the menu bar is hidden, the keyboard shortcuts set by the menu bar are disabled. They should always work, so this also adds a workaround for that.

launcher/ui/MainWindow.h

@@ -186,6 +186,10 @@ private slots:
 
     void globalSettingsClosed();
 
+    void keyReleaseEvent(QKeyEvent *event) override;
+
+    void keyPressEvent(QKeyEvent *event) override;
+
 private:
     void retranslateUi();