feat: animation pause and manually step to next frame

Resolve: https://github.com/BLumia/pineapple-pictures/issues/85

app/mainwindow.h

@@ -92,6 +92,9 @@ private slots:
     void on_actionPrevPicture_triggered();
     void on_actionNextPicture_triggered();
 
+    void on_actionTogglePauseAnimation_triggered();
+    void on_actionAnimationNextFrame_triggered();
+
     void on_actionHorizontalFlip_triggered();
     void on_actionFitInView_triggered();
     void on_actionFitByWidth_triggered();