HackStudio: Clear ToDo entries upon opening new project

The ToDoEntries and ToDoEntriesWidget classes now have methods for
clearing the entries, before entries would stay permanently, even after
switching to a new project.

Userland/DevTools/HackStudio/ToDoEntriesWidget.h

@@ -18,6 +18,8 @@ public:
 
     void refresh();
 
+    void clear();
+
 private:
     explicit ToDoEntriesWidget();