feat: support custom shortcut for existing actions

Resolve https://github.com/BLumia/pineapple-pictures/issues/128

Actually also https://github.com/BLumia/pineapple-pictures/issues/72
but not sure why that one is now a 404 page.

app/settings.h

@@ -46,6 +46,10 @@ public:
     void setInitWindowSizeBehavior(WindowSizeBehavior wsb);
     void setHiDpiScaleFactorBehavior(Qt::HighDpiScaleFactorRoundingPolicy hidpi);
 
+    void applyUserShortcuts(QWidget * widget);
+    bool setShortcutsForAction(QWidget * widget, const QString & objectName,
+                               QList<QKeySequence> shortcuts, bool writeConfig = true);
+
 private:
     Settings();
 
@@ -57,4 +61,3 @@ signals:
 
 public slots:
 };
-