Fix window resizing to minimized size when maximized and fullscreen toggled

This fixes issue #38 (closed) The window was resized to its normal size instead of disabling Fullscreen mode

src/interface/Interface_graphic.cpp

@@ -644,10 +644,8 @@ void Interface_graphic::on_fullscreen_triggered() {
      The menu is checked before function call, so if 'fullscreen menu' is checked,
      the main window is displayed in fullscreen mode
      */
-    if (_ui_fullscreen_menu->isChecked())
-        this->showFullScreen();
-    else
-        this->showNormal();
+
+    this->setWindowState(this->windowState() ^ Qt::WindowFullScreen);
 }
 
 void Interface_graphic::on_vertical_line_triggered() {