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

Decomposed old StyleHelper into StyleManager, StyleRegistry and StyleMerger.

src/Spout/Writer/ODS/Manager/Style/StyleRegistry.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Box\Spout\Writer\ODS\Manager\Style;
+
+use Box\Spout\Writer\Common\Entity\Style\Style;
+
+/**
+ * Class StyleRegistry
+ * Registry for all used styles
+ *
+ * @package Box\Spout\Writer\ODS\Manager\Style
+ */
+class StyleRegistry extends \Box\Spout\Writer\Common\Manager\Style\StyleRegistry
+{
+    /** @var array [FONT_NAME] => [] Map whose keys contain all the fonts used */
+    protected $usedFontsSet = [];
+
+    /**
+     * Registers the given style as a used style.
+     * Duplicate styles won't be registered more than once.
+     *
+     * @param Style $style The style to be registered
+     * @return Style The registered style, updated with an internal ID.
+     */
+    public function registerStyle(Style $style)
+    {
+        $registeredStyle = parent::registerStyle($style);
+        $this->usedFontsSet[$style->getFontName()] = true;
+
+        return $registeredStyle;
+    }
+
+    /**
+     * @return string[] List of used fonts name
+     */
+    public function getUsedFonts()
+    {
+        return array_keys($this->usedFontsSet);
+    }
+}