HackStudio: Start working on a "Locator", much like Qt Creator has

Pressing Ctrl+K will now open the little locator command line at the
bottom of the window. Right now it can only be used to jump quickly
to a file.

DevTools/HackStudio/Locator.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class LocatorTextBox;
+class GTableView;
+
+class Locator final : public GWidget {
+    C_OBJECT(Locator)
+public:
+    virtual ~Locator() override;
+
+    void open();
+    void close();
+
+private:
+    virtual void keydown_event(GKeyEvent&) override;
+
+    void update_suggestions();
+
+    explicit Locator(GWidget* parent);
+
+    RefPtr<LocatorTextBox> m_textbox;
+    RefPtr<GWindow> m_popup_window;
+    RefPtr<GTableView> m_suggestion_view;
+};