Spreadsheet: Reorganise the sources a bit

This commit just moves some code around:
- Give Cell its own file
- Pull all forward-declared classes/structs into Forward.h
- Clean up the order of member functions a bit

Applications/Spreadsheet/Cell.h

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "Forward.h"
+#include "JSIntegration.h"
+#include <AK/String.h>
+#include <AK/Types.h>
+#include <AK/WeakPtr.h>
+
+namespace Spreadsheet {
+
+struct Cell : public Weakable<Cell> {
+    Cell(String data, WeakPtr<Sheet> sheet)
+        : dirty(false)
+        , data(move(data))
+        , kind(LiteralString)
+        , sheet(sheet)
+    {
+    }
+
+    Cell(String source, JS::Value&& cell_value, WeakPtr<Sheet> sheet)
+        : dirty(false)
+        , data(move(source))
+        , evaluated_data(move(cell_value))
+        , kind(Formula)
+        , sheet(sheet)
+    {
+    }
+
+    void reference_from(Cell*);
+
+    void set_data(String new_data);
+    void set_data(JS::Value new_data);
+
+    String source() const;
+
+    JS::Value js_data();
+
+    void update(Badge<Sheet>) { update_data(); }
+    void update();
+
+    enum Kind {
+        LiteralString,
+        Formula,
+    };
+
+    bool dirty { false };
+    bool evaluated_externally { false };
+    String data;
+    JS::Value evaluated_data;
+    Kind kind { LiteralString };
+    WeakPtr<Sheet> sheet;
+    Vector<WeakPtr<Cell>> referencing_cells;
+
+private:
+    void update_data();
+};
+
+}