HackStudio: Make FindInFilesWidget reset when opening new project

Add method to FindInFilesWidget which resets the model in order to
clear the output when needed, and use this when a new project is opened.

Userland/DevTools/HackStudio/FindInFilesWidget.h

@@ -20,6 +20,8 @@ public:
 
     void focus_textbox_and_select_all();
 
+    void reset();
+
 private:
     explicit FindInFilesWidget();