TextEditor: Move the application UI into a dedicated TextEditorWidget. (#292)

Added a main widget for the text editor as a stepping stone to add new features.

Applications/TextEditor/TextEditorWidget.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <AK/Function.h>
+#include <LibGUI/GApplication.h>
+#include <LibGUI/GTextEditor.h>
+#include <LibGUI/GWidget.h>
+#include <LibGUI/GWindow.h>
+
+class GTextEditor;
+
+class TextEditorWidget final : public GWidget {
+public:
+    TextEditorWidget();
+    virtual ~TextEditorWidget() override;
+    void open_sesame(const String& path);
+
+private:
+    GTextEditor* m_editor{ nullptr };
+    String m_path = "/tmp/TextEditor.save.txt";
+};