Adds ability for editor-xtd plugin to generate multiple buttons

This creates the ability for an editor-xtd plugin to return an array of buttons, instead of just one button object.

### Testing
To test, replace this code in
/plugins/editors-xtd/readmore/readmore.php
```
return $button;
```

with:
```
		$button2 = new JObject;
		$button2->modal = false;
		$button2->class = 'btn';
		$button2->onclick = 'alert("Hurray!");return false;';
		$button2->text = 'An extra button!';
		$button2->name = 'smiley-2';
		$button2->link = '#';

		return array($button, $button2);
```

libraries/cms/editor/editor.php

@@ -472,10 +472,20 @@ public function getButtons($editor, $buttons = true)
 			}
 
 			// Try to authenticate
-			if (method_exists($plugin, 'onDisplay') && $temp = $plugin->onDisplay($editor, $this->asset, $this->author))
+			if (!method_exists($plugin, 'onDisplay'))
 			{
-				$result[] = $temp;
+				continue;
 			}
+
+			$button = $plugin->onDisplay($editor, $this->asset, $this->author);
+
+			if (is_array($button))
+			{
+				$result = array_merge($result, $button);
+				continue;
+			}
+
+			$result[] = $button;
 		}
 
 		return $result;