Row objects and Cell styling

This commit introduces Row and Cell entities, that will replace the arrays passed in previously.
It also adds support for Cell styling (instead of Row styling only).

tests/Spout/Writer/Common/Manager/RowManagerTest.php

@@ -0,0 +1,86 @@
+<?php
+
+namespace Spout\Writer\Common\Manager;
+
+use Box\Spout\Writer\Common\Creator\Style\StyleBuilder;
+use Box\Spout\Writer\Common\Entity\Cell;
+use Box\Spout\Writer\Common\Entity\Row;
+use Box\Spout\Writer\Common\Manager\RowManager;
+use Box\Spout\Writer\Common\Manager\Style\StyleMerger;
+use PHPUnit\Framework\TestCase;
+
+class RowManagerTest extends TestCase
+{
+    /**
+     * @return void
+     */
+    public function testApplyStyle()
+    {
+        $rowManager = new RowManager(new StyleMerger());
+        $row = new Row([new Cell('test')], null, $rowManager);
+
+        $this->assertFalse($row->getStyle()->isFontBold());
+
+        $style = (new StyleBuilder())->setFontBold()->build();
+        $rowManager->applyStyle($row, $style);
+
+        $this->assertTrue($row->getStyle()->isFontBold());
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderForTestHasCells()
+    {
+        return [
+            // cells, expected hasCells
+            [[], false],
+            [[new Cell('')], true],
+            [[new Cell(null)], true],
+            [[new Cell('test')], true],
+        ];
+    }
+
+    /**
+     * @dataProvider dataProviderForTestHasCells
+     *
+     * @param array $cells
+     * @param bool $expectedHasCells
+     * @return void
+     */
+    public function testHasCells(array $cells, $expectedHasCells)
+    {
+        $rowManager = new RowManager(new StyleMerger());
+
+        $row = new Row($cells, null, $rowManager);
+        $this->assertEquals($expectedHasCells, $rowManager->hasCells($row));
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderForTestIsEmptyRow()
+    {
+        return [
+            // cells, expected isEmpty
+            [[], true],
+            [[new Cell('')], true],
+            [[new Cell(''), new Cell(''), new Cell('Okay')], false],
+        ];
+    }
+
+    /**
+     * @dataProvider dataProviderForTestIsEmptyRow
+     *
+     * @param array $cells
+     * @param bool $expectedIsEmpty
+     * @return void
+     */
+    public function testIsEmptyRow(array $cells, $expectedIsEmpty)
+    {
+        $rowManager = new RowManager(new StyleMerger());
+
+        $row = new Row($cells, null, $rowManager);
+        $this->assertEquals($expectedIsEmpty, $rowManager->isEmpty($row));
+    }
+}