Make menus and placeholders display appropriate custom key combos. (#4045)

* Add initial support for finding hotkey display key sequences

* Make neededArguments work

* Implement displaying key combos in SplitHeader main menu

* Make Settings search text dynamic

* Make tab hide notice use a custom hotkeys key sequence

* Make Notebook menus use custom hotkeys key combo lookup for hiding tabs

* shut up changelog ci

* Make NotebookTab menus show custom hotkeys. SCUFFED:
this does not update dynamically!

* Scuffed: Make the show prefs button setting show the key bind

* Scuffed: Make the R9K description refer to hotkeys

* @pajlada, is something like this ok?

Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

src/widgets/Notebook.hpp

@@ -86,6 +86,7 @@ protected:
     }
 
 private:
+    void updateTabVisibilityMenuAction();
     void resizeAddButton();
 
     bool containsPage(QWidget *page);
@@ -111,6 +112,7 @@ private:
     bool lockNotebookLayout_ = false;
     NotebookTabLocation tabLocation_ = NotebookTabLocation::Top;
     QAction *lockNotebookLayoutAction_;
+    QAction *showTabsAction_;
 };
 
 class SplitNotebook : public Notebook