Added ability to toggle visibility of tabs (#2600)

This can be done by right-clicking the tab area or pressing the keyboard shortcut (default: Ctrl+U).

Co-authored-by: Leon Richardt <leon.richardt@gmail.com>
Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/widgets/Window.cpp

@@ -401,6 +401,10 @@ void Window::addShortcuts()
             new QuickSwitcherPopup(&getApp()->windows->getMainWindow());
         quickSwitcher->show();
     });
+
+    createWindowShortcut(this, "CTRL+U", [this] {
+        this->notebook_->setShowTabs(!this->notebook_->getShowTabs());
+    });
 }
 
 void Window::addMenuBar()