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/helper/NotebookTab.cpp

@@ -73,6 +73,12 @@ NotebookTab::NotebookTab(Notebook *notebook)
                          this->highlightEnabled_ = checked;
                      });
     this->menu_.addAction(highlightNewMessagesAction_);
+
+    this->menu_.addSeparator();
+
+    this->menu_.addAction("Toggle visibility of tabs", [this]() {
+        this->notebook_->setShowTabs(!this->notebook_->getShowTabs());
+    });
 }
 
 void NotebookTab::showRenameDialog()