Refactory Writer Styles to match new code organization (#433)

Decomposed old StyleHelper into StyleManager, StyleRegistry and StyleMerger.

tests/Spout/Writer/ODS/Manager/Style/StyleRegistryTest.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Box\Spout\Writer\ODS\Manager\Style;
+
+use Box\Spout\Writer\Common\Creator\Style\StyleBuilder;
+
+/**
+ * Class StyleRegistryTest
+ *
+ * @package Box\Spout\Writer\ODS\Manager\Style
+ */
+class StyleRegistryTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @return StyleRegistry
+     */
+    private function getStyleRegistry()
+    {
+        $defaultStyle = (new StyleBuilder())->build();
+        return new StyleRegistry($defaultStyle);
+    }
+
+    /**
+     * @return void
+     */
+    public function testRegisterStyleKeepsTrackOfUsedFonts()
+    {
+        $styleRegistry = $this->getStyleRegistry();
+
+        $this->assertEquals(1, count($styleRegistry->getUsedFonts()), 'There should only be the default font name');
+
+        $style1 = (new StyleBuilder())->setFontName("MyFont1")->build();
+        $styleRegistry->registerStyle($style1);
+
+        $style2 = (new StyleBuilder())->setFontName("MyFont2")->build();
+        $styleRegistry->registerStyle($style2);
+
+        $this->assertEquals(3, count($styleRegistry->getUsedFonts()), 'There should be 3 fonts registered');
+    }
+}