HackStudio: Move "find in files" widget to its own file/class

Instead of clogging up main.cpp with find-in-files functionality,
put it in a FindInFilesWidget class in a separate file.

DevTools/HackStudio/FindInFilesWidget.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GButton;
+class GListView;
+class GTextBox;
+
+class FindInFilesWidget final : public GWidget {
+    C_OBJECT(FindInFilesWidget)
+public:
+    virtual ~FindInFilesWidget() override {}
+
+    void focus_textbox_and_select_all();
+
+private:
+    explicit FindInFilesWidget(GWidget* parent);
+
+    RefPtr<GTextBox> m_textbox;
+    RefPtr<GButton> m_button;
+    RefPtr<GListView> m_result_view;
+};