HackStudio: Start adding a "find in files" function

Projects now contain a set of TextDocument objects. Each TextDocument
represents a member file in the project. TextDocuments may not have
their file contents loaded at all times, but they will be loaded on
demand when calling TextDocument::contents().

"Find in files" works by iterating over the documents in the project
and calling find(needle) on each one. The return value from find() is
a vector of line numbers where the needle was found.

This is obviously going to need a bunch more work. :^)

DevTools/HackStudio/Project.h

@@ -1,6 +1,8 @@
 #pragma once
 
+#include "TextDocument.h"
 #include <AK/Noncopyable.h>
+#include <AK/NonnullRefPtrVector.h>
 #include <AK/OwnPtr.h>
 #include <LibGUI/GModel.h>
 
@@ -12,10 +14,18 @@ public:
 
     GModel& model() { return *m_model; }
 
+    template<typename Callback>
+    void for_each_text_file(Callback callback) const
+    {
+        for (auto& file : m_files) {
+            callback(file);
+        }
+    }
+
 private:
     friend class ProjectModel;
     explicit Project(Vector<String>&& files);
 
     RefPtr<GModel> m_model;
-    Vector<String> m_files;
+    NonnullRefPtrVector<TextDocument> m_files;
 };